It is setup this way by design, to extend past 24 hours when the voting significantly changes. There are several reasons for this. The main one is to prevent gaming of the system. If it was a set 24 hours and a malicious whale wanted to upvote content that didn't deserve a payout without giving other whales a chance to downvote, they could do it right at the end of the timer. It also gives posts that get late attention from whales more opportunity to pick up more votes once it is discovered.