You have identified one of the great problems in course accuracy.
The vast majority of measurers do a great job The major events only use the top measurers. But unless the measurer is onsite, there will always be the question of the course being set up properly. And even if they are, there are situations that are out of the control of the race organizers such as illegally parked cars, or last minute construction.
I remember at the 1996 Olympics there were a row of TV trucks parked outside of the Olympic Stadium directly on the marathon route that added a few meters to the course. Nothing we could do about it!
All the measurers I know who attend the event are very conscientious, they check the course to make sure every cone and distance marker is accurately placed.
One of the problems for a measurer is knowing what parts of the roadway will actually be available and then matching that to the needs of the event (number of participants and other logistical needs). And then a race might have the situation that on race day, the police won't allow the usage of certain parts of the road (lanes)beacuse of various reasons.