I think people some people say that just because they're worried about the results.
I found some of my GCSE subjects quite difficult (English and Geography mainly). Of course I revised for them - but at any point between starting revision and finally getting the results, if you had asked me how much I had revised it would have always felt like not enough. Until I got the results, I was constantly thinking "should have revised more, should have revised more", and was obviously pleasantly surprised to get A*s.
So it was true - I had done hardly any revision compared to how much I felt I should have. It can be very difficult for a person to actually feel like they've done a lot/enough, if they're not too confident in a particular subject.
On the other hand, there were some subjects that I really didn't revise for (Maths, French and Spanish), because they came more naturally to me. Though for those subjects, I wouldn't have thought on results day "I don't know how I got an A*, I didn't know anything". I just found them to be more dependent on how much attention I'd been paying over the year rather than how frantically I crammed at the end.