Games : How come that by the time a new char reaches skeleton king they are about level 15 regardless of difficulty?

So I like leveling low level characters and I did that several times on different difficulties. What I noticed that when you reach skeleton king you are about level 15. It does not seem that difficulty setting matters. Expert gives 100% more experience compared with Normal, and still by Skeleton King you are the same level. How come?

If you go through the same content and getting twice as much xp, you should end up being higher level on the same point of story progression. But somehow that does not happen.