i'm very confused and i would like to understand why there's such diference between the two browsers.
Well, yes, you are. The best way to fix that would be to read and understand the CSS2 specification <http://www.w3.org/TR/REC-CSS2/>.
A summary is that what you are trying to do is not supposed to work.
The correct way to make it work is to use position: fixed; but IE does not support that yet.
Tim.
