In horse racing, how can the payouts be more than the betting pools?

This rarely happens, but because of the way of odds are rounded to the nearest 10 cent or 20 cent increment, when an overwhelming percentage of the money in the show pool is bet on a heavy favorite that can create what is known as a “minus pool.”
That means there isn’t enough left to meet the minimum payout threshold, either $2.10 or $2.20 depending on the state. In those instances, the track or advance deposit wagering provider is responsible for making up the difference.