When you are in RPFight, both Seal and Body Expand's boosts decay over time. I'd assume it'd occur with other boosts as well, but those are the only ones we have access to at the moment. I'd assume it'd have to do with how the game handles boosted stats (I'd assume it'd use absorbadd) and how it keeps the boosts from dropping over time. RPFight probably disables whatever it is that keeps the boosts from decaying, so when you're in RPFight, it treats static boosts like Absorb's temporary boosts, the ones that normally decay over time.