Can you put this in Codepen?
However, my initial guess is because you are using absolute positioning.
It’s just my opinion but absolute position is usually a BAD idea unless you are trying to achieve something specific.
The are better, more flexible and easier to maintain ways of laying out your page.
Also, this is because a declared pixel is not really a screen pixel :http://www.quirksmode.org/blog/archives/2010/04/a_pixel_is_not.html
Eventually the way the browser will run out of space trying to cram all the pixels you have declared into a set width and the page breaks.