I think Trunks is being cautious... Like, even though Cell may be behind Goku, ganging up on him allows any chance Cell may have of getting away. Or he thinks Cell has more power in reserve. Like, if he senses Cell as considerably below 100% MSSJ Goku and sees Cell encouraging Goku to get a Senzu Bean, and thinks Cell is not stupid, he may think the logical thing is that Cell hasn't shown FP. I'd say, though, while Cell has lot a lot of power as he said himself, he's still above everyone there even damaged and not powering up, sans Goku and Gohan... But Goku has already fallen to a level way below Vegeta/Piccolo/Trunks... As per the Cell Juniors' performance... And I don't believe Goku continued to lose power.
Cell (vs Goku) 100
Goku 90
Cell (post KMHMH) 88
Goku (post KMHMH) 80
Cell (post chi blasts) 75
Goku (post chi blasts) 35
Vegeta/Trunks/Cell Juniors 60
Piccolo 50
Something like this, I guess.