Cell seemed to be one of those characters that needed a demonstration before being convinced he was weaker.
-Piccolo powered-up and Cell still tried to fight him directly. He only realized he was outmatched after getting knocked around by him
-Semi-Cell thought Super Saiyan Vegeta challenging him was a joke even though Vegeta was strong enough to fight him at that level
-Cell thought his full-power was more than enough to handle Gohan
Goku looked a bit taken aback by Cell's full-power:
You almost get the impression that Goku was expecting the fight to be much tougher for Gohan. Of course the smile returned after Gohan dropped him with ease. It's possible that Gohan standing around doing nothing might've made it tough for him to understand his real capabilities. Then again, Cell had no issue sensing that Super Saiyan Grade 3 Trunks was much stronger than himself.
Cell's sensing sorta comes and goes. Kinda reminds me of how Super Boo was smart enough to gauge Gohan from the Kaioshin world, only to be shown as a complete dumbass (with Piccolo absorbed) in understanding that Super Vegito was way ahead of him.