In the manga I more or less got the feeling it was like
Piccolo- Holy crap Goku is strong now... He and presumably Cell have a power that seems to be untouchable.
Piccolo- (As he is nearing 50% Goku's power) I'm getting stronger. Perhaps I will have a chance to play a role in the upcoming Cell Game. However, knowing Goku, he probably has more in the tank. Hmmm, come to think of it, he probably has a lot more in the tank. Damn you Goku for always making me so irrelevant.
And I think Piccolo meditates on that thought while training until he comes to the realization that he is indeed garbage, but he only believes it 90% of the way. He comes out and acts cocky like he knows, but he was hoping he was wrong.
Trunks complete shock shows that he suspected nothing.
Trunks probably tells Vegeta later who dismisses the notion, but the fear of doubt creeps in over the last 10 days and it starts to dawn on him that they are probably right. Which is why he has faux confidence heading into the Cell Game and acts cocky, but he knows deep down, Goku has his beat once again.