GPS trackers aren't everything, nor are they perfect. I walk the same path everyday at work (unless it's raining) and get a different distance every time. Recently the path is consistently 0.2 of a mile shorter. Consistently, but not always. These trackers are easy and give you lots of geeky stats, but only if you just trust them to be close enough. Much like counting paces, using a timer, and Map Your Run with a running journal.