Endomondo can be really creative about distances. It told me that my "Peachtree Qualifier" 10-k was about half a mile short.

Someone else told me that courses always are laid out so that the minimum possible distance you can run if you stay right against the inside on every turn and never zig zag is at or over the stated distance or no record would ever be recognized. So if you meander a little and swing wide on many of the turns you will be over a noticeable amount.