I guess that Hilton website is supposed to be the lowest at the time when you reserve the room. It probably has some kind of script checking prices constantly. Now if you come back after you bought the room (without 24 hours), other sites could have lowered their prices. And to avoid paying you the $50, instead of live prices, they use database of the prices 1 day ago at the time of your purchase! Since they have already done the checking at the time of your reservation, it would ensure that they will never have to pay the $50.