My experience with my GPS in my iphone is that it will cut corners. I have mapped runs with mapmyrun.com said it was 4 miles go out run it gps says it was 3.6 miles. go to runkeeper's website look at the data make it follow the roads and come up with 4 miles. so either the roads (I believe they both use google maps) are mapped horribly wrong, or the gps is cutting the corners. like you take a left the last point before the left and the first point after the left are connected cutting out distance. you make a good amount of turns and go through a good amount of curves and it ads up.