Sorry if this is long and tedious, but what the heck else is there to do?

In terms of tire width, I think there are 3 main reasons for inconsistency:

1. Lack of Standards: Actual tire width is highly dependent on rim width---actual tire width increases on wider rims. So the marked width number is semi-meaningless without knowing the rim width used to calculate that measurement. But there doesn't seem to have been any industry standard on what rim width to use to measure/state the tire width. that leads to lots of confusion with the wide range of rim widths in the market today. Is a 700x28 Continental GP5000 tire 28mm on a 1986 Mavic Open Pro rim with 15mm inside width, or a current Mavic Open Pro, with 19mm inside width? What happens on a HED Belgium+ rim at 21mm internal width? The tire mfrs don't tell us, so we have no way of knowing except through trial-and-error, or the interwebs.

With the introduction of tubeless tire/wheel systems, and tire/rim sizing/fit getting really critical, there is a new standards scheme brewing. Lotsa info out there on the web, I tried reading some and got a headache. I once buttonholed a Panaracer rep at a show and suggested tire sizes should be listed as diameter-tire width-rim width, so we'd know. In my perfect world, a 700x28 tire that was 28mm on a 19mm internal-width rim would be labeled 622-28-19. Then we'd know how much narrower/wider it might be on a 17mm/21mm rim. [sarcasm]I'm sure that suggestion went right to the top.[/sarcasm]

But what happens when the labeled width is "wrong?" That leads to reason #2 :

2. Manufacturing Inconsistencies: Tires are made with molds, and for industrial reasons I don't at all understand, it's evidently not unusual for a tire to pop out of a mold wider or narrower than intended. They're not going to throw out the mold when that happens, and the size/width labeling is molded into the rubber, so the tire will remain that "inaccurate" size until the mold wears out. "Inacccurate" in quotes, because, you know, the tire will likely be the labeled width on some rims----we just don't know which.

And when the mold is replaced, it will likely be replaced by an identical mold, with the identical tire width popping out.

Examples of this are legion. The Conti GP4000SII 28mm tire was famous for plumping up to 32mm on wider rims, so much so that when Conti introduced the 5k, they added a 32mm size, and made the 28mm narrower.

I suspect the GP4k/28 was a surprise when it popped out of the mold, and Conti never bothered to change it. I remember the original Grand Bois Cypres was labeled 700x29, but was usually 32mm+ on common rims at the time. To Grand Bois/Panaracer's credit, they did eventually change the molded width to 32.

3. Marketing? I'm not fer sure about this, but when suddenly wider tires started to get popular, there was a loooong string of years where Panaracer Pasela tires were ridiculously labeled undersized, on the common rims of the day. 700x32 tires were like 27mm. My memory's hazy on this now, didn't take me long to stop using them 20yrs ago, but it seemed pretty consistent across the line, every Pasela was 5mm narrower in real life---so it felt more like a marketing exercise than production errors. The Paselas were eventually changed, and when folks were selling them there were always questions about which label they had, the old or new. Because the old-label tires were stupid narrow.

I kinda feel like there were other examples, from other mfrs as well. I maybe kinda remember there were tires with one width molded in, but a wider width shown on the hot-patch label. But I'll leave it with a ??? since who knows? And this far removed in time, it doesn't matter much, except if one of those tires is in a vintage frame and you're trying to figure out what actual width might fit. Hint: never trust the label, always measure the actual width.

Tire and rim actual diameter matter for mounting, and that's maybe a whole other topic. Likely similar issues there.

Tire diameter must also be dependent on the tire molds. There's a known, definite standard for diameter, but I don't think there's any standards body checking/enforcing that. Likely up to mfrs' QC staff?

I remember a run of cheapo tires on new Fujis/Panasonics in the early '80s that were fine up to the labeled 75lbs recommended max pressure. One pound over that and they'd blow right off, regardless of rim. I think tires/rims are supposed to be able to got to twice the highest recommended pressure before blowing off, but again, who enforces that? Fuji replaced all the tires for us, Panasonic didn't.

And rims are just long extrusions rolled and welded/pinned to make a hoop, like a neato Play-Doh Fun Factory toy. If the Play-Doh rim extruder is off by a few fractions of a mm, you've got a nightmare rim that is either holy heck to mount, or lets the tire blow at low pressure. I think the blow-offs get culled by QC, but the nightmare-tight ones often get shipped. I watched some rims being extruded, I mentioned that folks were complaining about tight tire fits, and was told basically: "Yeah, we were rolling them a little too long, but we took care of that." The "a little too long" ones shipped on hundreds of thousands of bikes.

What it comes down to for me, an old guy raging against aging, who wants to cram in as wide a tire as possible on everything, is I have to have a sheet-ton of tires around, and it helps immensely to get familiar with which tires are accurate on narrow rims, and which on wide rims. And I have to hope for consistency.

I use a lot of Soma Supple Vitesse tires, with the very nice high-zoot Panaracer supple casings, and cheaper than Compass/Rene Herse. They are what you might call undersized, meaning they seem to hit their labeled width on wider rims. So a 33mm Soma SV will be 33mm on a wide-ish HED Belgium/+, so probably range 29-31mm on vintage/narrower rims. Compass/Rene Herse are similar.

Grand Bois tires I've used, 28/32, and newer Vittoria Corsa+ tires in 28/30 are kinda oversized, they're close to labeled width on narrower rims, so they'll plump up wider on wider rims.

I've used some Conti 5k in 28/32, and they seem to be "undersized," accurate on wider rims, narrower than labeled on narrow rims.

I haven't used many Panaracer-labeled tires in a while, lotsa Panaracer-made tires, but with other folks' labels. Not sure what's happening with them width-wise in general these days, though I do know folks happy with some of the slick Gravel Kings on vintage bikes. I did snag a couple pair of uber-light Japan-market Panaracer Competition SX Protite 700x28 tires on the cheap, and they're plump, 30mm on wider rims.

If you're not sweating every last mm, and/or you're not as retentive as stunod me, you can just scratch your head and not worry too much. But if you have a frame that, fer instance, is too tight for comfort with 28mm actual width, and you really want 26mm i/o 25mm, 'cause, ya know, at narrower widths every mm counts, then what you've got is a headache.

What's maybe working for us moving forward are new tire/rim sizing standards. What's working against us is the prevalence of tubeless systems, which need unholy tight-fitting tires/rims to seal in the sealant.