WOW,
I may not completely understand your post reference because I am not all that smart I will be honest. LOL
When I used to ride in groups we all use a different devices of same brands, different brands and compared numbers. What we find is even if we start our GPS at the same point, we rarely have the same distance. Over a 20-30 mile ride we would sometimes at worst be off by .2 of a mile. Usually we are at .1 or closer though. I don't think that is significant at all, close enough.
I use a wheel sensor with my Garmin 1030 and I feel like my numbers are pretty consistent and accurate being I ride the same routes a lot and usually very close.
I have only once rode with two different GPS brands at the same time, no sensor, using only GPS and after about 25 miles they were within .02 miles that one time.
I don't worry about .1 or .2 mile differences. But I don't have a need for it to be perfect.
Sorry if my ramblings are no help to your question......I really don't under GPS all that well or how it works I guess.