Because the money used for purses is not NYRA's money. It belongs to the horsemen as part of the negotiated contract concerning the takeout from handle and other sources of revenue. Occasionally the tracks will "overpay" purses where they use more than the alloted amount and have to reach into their pockets to front the horsemans account money.
Because they run a year round schedule I would gather that NYRA never overpays. I was just answering the question in general. But in the case of an overpayment the track would get the money back from the horsemans account once it had built back up.