It is inherent in cross country but not so much in road races. That's why we have USATF course certification in the first place. So we can know to within a good degree of accuracy how long the course is.
Most people in your standard road race dont care about the people around them. If they know somoen personally thats different but most of the time, the people around you are just faces in the crowd. Seriously, what difference does it make if you finish 178th or 177th? They are trying to run FAST. In a certified race one might make allowances for the terrain but not for the distance. If they enter a certified marathon they want it to be within certification-level accuracy of 26 miles 385 yards.
That said, if you have a certified map and the course is set up right and the Garmin says it's wrong, then turn off the damn garmin, its margin for error is much bigger.