One Wouldn't Think It Hard To Measure

Color me Rad "5k" run was this past weekend in Philly.  It was fun, but it was short...very short...maybe 2.5 miles instead of 3.1 short.  I realize that it was a fun, untimed run, but is it really that hard to lay out a course that clocks in at (close to) 5k?  It can be done fast and free online, at places like this or someone could have walked it with a decent gps device.  As easy as it is to do this, and as much as it said "5k" throughout, that the race was about a full kilometer short was just...weird.