Some people think Piccolo overheard the conversation between Goku and Karin due to his superior hearing, but if that were the case he'd know Goku only showcased 1/2 of his powers and wouldn't be shocked at Goku displaying his full power at the Cell Games considering he'd be already expecting it imo. All of them were shocked because they still underestimated the length of Goku's powers.
But Piccolo definitely thought that what Goku showed at Karin wasn't his full power after his RoSaT trip, which is why he deems himself trash to Cell, but he still probably didn't think Goku's FP had a whooping 2x gap over the one he showed earlier.
Goku [50%] - 50
Goku [Piccolo's estimations] - 75
Cell [Piccolo's estimations] - 82.5
Goku [actual power] - 100
Cell [vs Goku] - 110