That is because there is a lead-in before the route/lap starts. Many routes are loops, almost all of them, so you essentially have to ride to the beginning of the loop before it starts. Since all spawn points are slightly different the distance to the beginning of the route can also be slightly different. Therefor the lead-in is not counted in the length of the route. This catches most new users by surprise, look for the blue line on the route selection screen, the blue line represents the lead-in.