Never said that it couldn't happen. Because based on the fact that I view human physical and mental development like snowflakes, infinite interations on a basic pattern yet, all unique, I'm sure there are very few absolutes in our reactions and psychologies.
It's just that when people claim to be able to tickle themselves, its always turned out to be 'surprise stimulation' or 'predictive reactions' (Predictive reactions are people who have an idea how a ticklish person reacts, and then acts in that matter, to the point they convince themselves the reaction is to the tickling, and not to the expectation) The schizophrenia example you pull above is a text book example of this. We are very able to make ourselves believe we are able to do many things. It's why a non-ticklish person under hypnosis can become so ticklish they can't stand up. Mind trumps body.
I don't doubt that you feel what you think you feel. But accept that your mind might be pulling the wool over your own eyes as on possible reason why.
Exceptions to physical behavior rules are super rare. They are either honest mutations, or psychological self manipulation as a rule.
But whatever way it is. You get the effect, and can enjoy it. And that is what counts.
Myriads