People have pretty much explained it but in a bit more detail Hulk and Banner still exist in each other when they are not in control and still have some impact on how the other behaves. During Planet Hulk they both agreed that Banner couldn't survive on that planet and that it was best for both of them that Hulk remained in control. Since they both felt this way and Banner didn't resist Hulk this is also why the Hulk is smarter as he can access some of Banners intelligence. An example of the opposite of this situation can be found in the Son of Hulk miniseries were just after World War Hulk Banner is fighting back and trying to suppress the Hulk which makes the Hulk loose all of his intelligence and become a dumb brute.
This blog also goes into a lot of detail explaining this.
http://www.comicvine.com/myvine/rasarima/hulk-s-tranformation/87-76837/
Also read the book it's great!
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