Ric's point makes a lot of sense. For some people the race may be their only one for the year or ever whereas regular runners know there is always another chance just around the corner.
Experienced runners generally know the difference between "good pain" and "bad pain"
A lot of people don't take heat in consideration when working out their pacing strategy. They may have been able to hold PB pace on a cold spring day but forget the effect that the heat will have so go out too fast for the conditions.