If the site or application you use to determine your distance only uses the plots in the log file, then remember that the log files may not have that coordinate that takes you to the furthest distance into the cul-de-sac. As well around sharp corners, it might miss actual corner where you turned.

If you have 1 second recording intervals for your log, then that can still be 20 or 30 feet missed during that one second. Many devices by default are doing more than 1 second interval logging. Some Garmin's by default do what they call "Smart Recording". I've seen enough differences in my logged track with that to know there is nothing smart about it.