There's nothing mathematical about the GPS. I think the most frustrating thing is that the unit can be off by as much as 10 meters in any direction, and that tall buildings and canyons can affect it as well (it's gotta be able to see 3 satellites at the least)
That said, we adore our GPSes and never travel without 2 handhelds and at least one laptop attached GPS. Makes trips much more fun!
If you want to be certain that you've got a reasonably good GPS reading, find a spot near the item of interest where it can see the full sky and let it sit for about 3 minutes. It'll tweak and adjust as the satellites move, but at the end of that time you should have something pretty close to what Google Earth has.
