In the end, all that matters is the distance the GPS reports at the end of the ride. I've seen some cases where the stored track doesn't look all that good as far as following my true path around turns and such, and yet the final distance traveled that the GPS reports was very close (within 0.2-0.3%) to the Jones counter distance calculated on the same ride.
These units are almost certainly not calculating distance by simply "connecting the dots" on the track points that they have saved for you. It's much more likely they are taking position readings much more often internally and using that information to calculate the distance you traveled. Once they have done that to calculate the distance traveled for the first say, 5 seconds, of your trip, they can discard that data and start taking readings for the next 5 seconds. They can't save all those readings in your track log though because it would take take too much memory.
It's very easy to do a careful test of your GPS accuracy. Do a set of calibration rides to get the cal constant for your Jones. Then go for a ride with your GPS. You don't have to follow a certified course or follow the SPR, just ride. But no backing up, no getting off your bike, and no freezing your front wheel and walking. When you finish your ride calculate the distance based on the Jones counts for the ride and compare to the distance your GPS reports.