I know, it's a potential way to explain the difference in performance without him actually increasing power.
He did say he would only go "all out" in speed. He can still increase his power without there being a contradiction, though I don't see the need for him to have increased his power either.
His attacks were completely ineffective, all that changed was that he could hit. If he increased his power, it would have been after his initial attack against Gohan, and even then it's not necessary. Full nelsons gave #16 the power to restrain Cell, Cell restraining Gohan in the same manner should be very easy, even with a large power difference.
Upon further review of the scene, I think I've reconsidered. Still, there is a point where the battle changes. His initial attack was useless and did nothing to Gohan, despite landing multiple solid hits. After that initial scuffle, his hits seem to become more effective.