about halfway down in this thread we did a pretty good discussion of "how GPS works"

How much accuracy do you demand from your bike computer? - Page 2 - Bike Forums

worth a read with some technical details of GPS internals.

your computer is calculating a waypoint every second and then "stitching" those points together to derive speed and distance

the computer does "snap" the position to a road because the raw GPS waypoint has a circular error of about 15 feet best case.

so there are a variety of errors that are possible

worth a read

/markp