It's as much conjecture on my part as it would have been if you wrote it, too. It's not like I design bicycle parts and whatnot for a living.

From a retailer's website:

Pasela is the standard all around touring or urban commuting tire. Its comprehensive size range insures that you can find a Pasela for most any usage. Ride it and trust it.

• 27" has different tread pattern than other 700c Pasela tires

Jobst Brandt's book says that spoke tension makes no difference on how the wheel handles bumps, that tire deflection is so much greater.

Interesting that this topic was presented. I am awaiting 2 pair of Sun M13II, one pair 700C and the other 27". Both will be laced to Campy Record HF hubs. I guess I can do my own reduced variable test. The bike will be a 1984 Trek 610.

There's a thread over on the CR list at the moment. Some fellow asking about recommendations for 27 X 1 1/4 rims. When I posted that I like the Sun M 13 II rims I have, two wheelbuilders there piped up and said they both thought the rim was junk. Problems with the joints, un-even tension, not flat, etc., etc.

I haven't noticed such problems with that rim. Though I have only built one of the two sets I have, and, I'm no pro wheel builder.
Anyone else have issues with the Sun rims?

That's not true, though.

I'm not sure you've got a 4 mm difference. You might have a 0 mm difference given that the 27" wheel has a 4 mm bigger radius, but a roughly 4 mm smaller tire. On the other hand, it's right after lunch, I'm full, kind of warm, and it could be that my logic/math portions of my brain are already asleep and waiting for the rest of my body to catch up.

another case solved!

Back in the day, I could sure tell the difference between a Coors and a Bud , blindfolded. But then, I cam of age in Utah, where , if you were lucky, the content came up to the max allowed 3.2. It all tasted foul to me. Like pee water. So I never developed a taste for beer.
Until I went to Beijing. :)
The only difference I can discern between wheel sets is between clinchers and tubulars.

When you start expanding beyond weasel piss, though... there are many drinkers who can distinguish between and among good beers. :)

A local bike shop mechanic (RIP, Mitch) built a 32 spoke 700c M13 II rim for me in 2010. I vary between 185 and more recently 200 lbs, and though I know I'm a bit heavy for the spoke count, I have had more truing issues than I would expect. The joint is off, too-- you can feel it in the braking and the anodizing there wears off pretty badly. Not a fan of that rim.

(with emphasis added)

Because, as you say, they were guessing. You got lucky!

I built with a 27" M12II then de-strung it because the joint waqs too uneven. I tried polishing it down but that didn't really help, jutws left it with a flat spot that still grabs the brakes. The LBS guy said it wouldn't stay that way long, but I gave up on it. However I have other M13IIs that are just fine.

I have M 13 II's that seem fine. Good joint, nice and tight. Seems to be aligned very well.

BTW, you can usually fix those mis-aligned joints, Standalone. Unless it's really bad. If they're anodized, though, it'll leave a mark.

These guys I referenced on the CR list were both pro wheel builders, it sounded like, so I didn't want to argue with them.
But the M13 IIs I have seem to be fine. Maybe I got lucky?

If the 28 mm tire was running a lot softer than the 32 mm tire, it could feel cushier.

Over this last Winter, I built sets using:
CR-18
M-13 II
CRT 16 II (NOS)
5 had a perceptible flat spot on the rim joint affecting radial true.
Not enough to really worry about. I only made minimal effort to remove it, striving for more even spoke tension instead.
I'm riding the M13's and never even think about it. I even hit an old style storm sewer grate the "wrong way" bouncing my 230 lbs. out of the seat about 8-10". About 5 minutes with the truing stand and tension meter and it was pretty much in the same tolerance as when new.
32 spoke.
14/15 DT Swiss DS
15/16 DT Swiss NDS.
Shimano RM-30 hub.

I built one rear using a Velocity Synergy OC I bought from Peter White. It had a similar flat spot, but cost about 3X as much as the Sun's.

The one guy recommended the Kin Lin 5 ALM rim. 27 incher.
Not that easy to find though.

I'm surprised noone has asked this yet, but what color are those tires? :D