There are errors associated with both GPS and wheel circumference odometers. GPS measures the distance between acquisition points which are never in a straight line. The "snap to" or smoothing feature in many GPS receivers does help as
@dodgeman stated above. At best, they are a series of straight lines which do not exactly follow the natural curves of a road or trail. This picture gives a visual representation of the effect:
View attachment 158425
The yellow center line of the road is the true distance measurement. The blue, red & green lines are GPS tracks made on four different trips along the road. The small squares are the acquisition points and the GPS distance is measured between them. It's easy to see why they don't match.
Wheel odometers rely on accurate circumference measurements based on the rolling radius of the wheel. This is very difficult to measure due to changes in surface conditions, tire pressure and rider position on the bike. You also have to take into consideration the minute distances added each time you turn the handlebars. No rider rides in a truly straight line.
Due to the difficulty in setting an accurate wheel circumference, I tend to agree with
@dodgeman that, under most conditions, the GPS measurements are likely more accurate.
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