wayliff: that's because GPS are only accurate to within 10m. So, if you have it running still when you're not moving, the signal that it gets from you can bounce around in a 10m radius. So one signal might come from 10m to the east of you, then another one might be 10m to the west, so it thinks you moved 20m. Furthermore, when you are moving, the 10m accuracy is only about 95%, so chances are your GPS is at least a few percentage points off from the true distance. It could be either over- or under-estimating the mileage, depending on a lot of factors. If you ever run a race with a certified course and wear a GPS, you will usually notice that the GPS is off by a few tenths of a mile and the mile markers don't line up because of GPS are inherently less than 100% accurate. They're still close enough that it doesn't really make that much of a difference; if it says you ran 4 miles and you really ran 3.97, your body can't tell that you shorted it.