Well, in real-world it might make some sense for Cell to go down in-terms of "power-stability" or durability, I mean he was forcefully making himself more powerful so it'd make more sense if he was concentrating his power to such a point that even the slightest of hits could make him go straight-back down to Normal Cell. Like if you were to lift weights to the highest amount you can, you can't let anything distract you. That kind-of thing.