The replay time will often be weird or off. I just rewatched one of mine. The first watch, it showed 29.90, the second time 30.23 and the third time 30.35. So yeah, it's somewhat random. Just don't rely on them, rely on your leaderboards score.
I think the map screen rounds them differently, but at least it's usually closer to the 'real' time than the replay time.
The reason this happens? Lag. The replays may show signs of speeding up the actions or slowing them down. Once the replay is fully processed it may show the right time.