Wow, thanks for all the replies. Lots of things to chew over...
The tire is a Kenda Kwick Trax K1053-003 700x32C
The 2173mm I think I got it by rolling a piece of masking tape around the tire and then measuring it. I guess a roll out measurement would be more accurate or even measuring 10 miles distance or so...
Also, all the things mentioned make sense. Front tire goes a longer distance, subtle differences in route, etc.
"Back when I used to care about such things".... maybe that's the best way to go. I'm not that obsessed with it to be honest, just did my first 100 miles the other day I was extremely pleased with myself. Traced my route on Google and that's how all this came about. I guess more curious than anything.
D