I think FDMaxey has explained it very well. :)
One other note is that I find American humour is often "signposted". That is, it is obvious when something is a joke and when something is not. British humour, by comparison, demands much more attention. I recently "tortured" a friend with The Office (British version - the Christmas special) and most of it was mysterious, even boring, to her. I was guffawing non stop.