It's an interesting topic and I agree with much of what has already been posted here.
I agree that most people have only their own experience to go on in terms of "how ticklish they are." Those of us who do a lot of tickling are somewhat better judges of comparable reactions, though we will still never know exactly how intense the feeling being tickled is of Person A compared to Person B.
I think it speaks to how difficult tickling is to bear even for those who, comparatively, are relatively "less" ticklish than others. You or I might be well aware, tickling someone, that she is somewhere in the average range compared to others we've tickled or seen tickled, but to that person, the tickling still feels unbearably intense.
The first time I tickled my college girlfriend she said, "I'm so ticklish....unbelievable..." Needless to say, that was very exciting to hear. The truth was that she was definitely ticklish, but nowhere near as ticklish as a lot of women I've tickled - probably middle/average range. She would squirm and laugh, but rarely laugh hysterically. Some of my favorite measures, doctors exams and pedicures, weren't that ticklish for her. But apparently the way it felt for her to be tickled was enough to make her think that she was "unbelievably ticklish."
One thing I'd add is that I think most genuinely extremely ticklish people know that they are extremely ticklish. They may not want to TELL YOU that....but they know it. Most such people were tickled a lot at various points in their life and even if they aren't tickled as much now, USUALLY those kind of accidental tickling scenarios - pedicures, doctors exams, massages, etc - don't let them forget how ticklish they really are. The one exception to that is that there are some people who are much more ticklish when they know someone is "trying" to tickle them, and therefore accidental tickles are unexpectedly manageable compared to how they react when intentionally tickled.