If you vote on a post before the 15-minute mark, you lose 15% of your rewards and they instantly go back to the reward pool, so there's nothing wrong with the first-minute voting.
For example, there's a whale who was making many, many comments every day on posts with little to no visibility. On the last day before payout, he'd upvote these comments, by which point the new comments would hide the old ones. For months, maybe even years, he got away with self-voting undetected because it was just hidden in the mess.
Not only that but if a post gets a huge upvote that attracts attention, doing it closer to cashout means there's less time for it to be noticed and flagged.
You have to ask yourself "Why would they repeatedly vote on old posts?"
Sometimes, it is just chance they've found a post they like that'll cash out soon. That's why I made the program, to spot patterns.