Fun With Math - Marathon Edition

OK class - here's one. How is it possible to have 20,600 finishers in a race that has 19,500 entrants? Want a hint. If you give out Tiffany necklaces as the finisher's medal as they did at the Nike Women's Marathon, so less than scrupulous women will bandit all or part of the course in order to collect the prize. Considering that of the original 19,500 there were presumably a few DNF's, it means that probably close to 1,500 women (or guys in drag) jumped onto the course at some point and finished the race. 

Then there's the tale of woe from one newbie runner who actually ran the whole thing but forgot her bib and was removed by the bandit catchers. Considering that they let well over 1,000 bandits sneak by, I'd suggest that the race organizers employ better bandit catchers next time around. I know one hypervigilant coach from NYC who'd be happy to do it in exchange for the free flight to San Francisco.