@OP: I'm not certain, but your opponent seems to be squandering vacation.
For example, if your opponent goes on vacation for an hour and then comes off vacation, his vacation is docked by one day. However, the 23 hours he was not on vacation can't simply vanish, so the time is added onto the time in his remaining games.
I would guess that your opponent is wasting his vacation time, which is very much to his disadvantage. He must run out eventually.
One of my online tournament games has reached a point where it is an easy win for me: I was already up a rook and just queened. Since then, my opponent is not moving, but he is not timing out. The time will decrease but then will increase again, say from 15 hours up to 28 hours. His online profile shows that he is logging on each day, and he never seems to be on vacation. This has been going on for several weeks and will at some point, if it continues, end up holding up the tournament.
Why does this happen? Can one use vacation time to add time back on to a game, or is this a bug, or what?