YES, poeple lie!!
I just got off the Wonder on Sunday and there were A LOT of sick people on the ship. Sitting in the Walt Disney Theater there was a constant chorus of coughing. A little boy threw up on the first night there two rows behind us and the woman commented that he had been sick for 2 days. Obviously, that means he was sick at the time they boarded and she knew it.
I pulled my kids out of the Mickey Pool one day due to an obvious sick child in pool area. The kid was OBVIOUSLY sick....I'd guess he was about 10 months old and his mother was holding him standing over the pool area watching her other child. The poor baby looked MISERABLE. Green junk oozing out of his eyes, which the mother kept wiping with a towel and then setting it down on the little ledge that all the kids stand on/touch to get off the slide.

She kept checking his forhead for fever and his cheeks were bright red. He was crying that poor miserable cry that made it clear he didn't feel well. The last straw for me, was when she was holding him right over the pool and the snot and gunk from his nose and eyes were just DRIPPING into the pool. She eventually saw it and tried to put her hand under his nose, but by then it was too late.
We saw this family multiple times in the common areas and every time the poor kid had green junk just oozing out of his eyes and nose, bright red cheeks and he was usually crying. His eyes were so swollen, crusty, red, that I don't think he could even see. I overheard the mother telling someone that the kid had been sick with a fever even before they got on the boat, but she thought he would get better. I totally get it that it would SUCK to have to miss the trip due to illness, but honestly, this poor baby was really sick and probably shouldn't have been there anyway. No telling how many kids have now have pink eye or some other infection from being exposed to this baby on the ship. I for one, wasn't thrilled.
So, based on some of the things I saw and overheard on the ship, I'm pretty sure people lie on those forms!!