After owning three cars that had more than 100k on the clock I have come up with a pretty simple idea of why some cars seem to show their mileage less.
The simple answer is money. If you go out and look at say cars with over 150k that run and look like new, most of them will probably be cars (or trucks) that were fairly expensive when new. The previous owner garaged the car, took good care of it, washed it regularly etc etc.
A good example of this is the Volvo I used to have. When I got rid of my 'tour it had a little over 130k on the clock. The Volvo I replaced it with was also a 96 and had over 100k too (although a little bit less at 105). The difference in the way the cars looked and felt for their mileage was night and day. Aside from the Volvo's somewhat dated look, most people didn't know it had over 100k and was 7 years old when I got it. The contour on the other hand surely looked it.
I wouldn't hesitate to buy another car with over 100k on the clock provided it looked and ran well, had the proper documentation etc. IMO, the years a car has under its belt can be more detrimental to its life than the mileage on the clock.
