I don't see how one would know this to be true unless they had been circumcised as an adult, right? Just from a physical standpoint, I don't see how removing skin would make something less and not more sensitive, but if that is what the literature says...
The skin on the head consists of mucous membrane, like the inside of your mouth, or the inside of your eyelids. All such mucous membranes are protected. When the foreskin is taken away, this mucous membrane is exposed to the air and to abrasion from clothes, so it hardens. Otherwise it would constantly get sore.
There is more to it... Check this article if you want further info:
http://www.foreskin.org/fleiss.htmDrastic solution to a yeast infection.
I guess people made up all those pro-circumcision pseudo arguments like improved hygiene because they wanted it to make sense that God told them to mutilate their male children...