If you check out the manual, or Garmin's website, I think they explicitly state that the thing is only accurate to within fifty meters for altitude. Maybe that means it's only accurate within fifty meters horizontally as well. 20 seconds at 6:00 pace is 89 meters which sounds like more than we'd like to see it be off by. However, I've found a few trends with my devices. They tend to track more accurately on my bike. I believe, though don't know, this has something to do with the fact that I'm generally going faster on my bike. It's not as accurate when you are going super slow. To prove this, go outside and turn your device on, once it gets it's signal hit "start" and don't move an inch. You'll see the display shows that you are moving, and maybe after a minute it will think you've moved 20-100 feet though you havn't moved at all.

If you are wearing the device on your wrist while running, your device may have actually moved through a space of one mile, even though your core did not. If you run one mile, your wrists and ankles have actually moved through space farther than one mile because they are swinging back and forth. Maybe this has something to do with it. Sometimes when I ride, I'll put the unit on my helmet if I'm in the mood to just feel a workout through from start to finish and don't want numbers in my mind. As I usually find myself working out in the forest, I've found that running with the GPS on my helmet tends to get me better reception. The only drawback is that most people will be very much self-conscious of running with a helmet on, especially if you're doing a brick and just run with your cycling bibs/jersey on. People have actually stopped and asked if I was OK, which makes me just smile real big and ask them "How do you mean?" It's actually quite comfortable though, and it keeps my head warm when it's cold out.