Having created a fairly simple site based on a commercial skin I've noticed that some of my pages are rendered as one of two specific widths. I found that by adding blank lines to a standard HTML module to increase the page height beyond a certain threshold, would change the rendered width of the page. As an example, I created one page with no content on it apart from one blank HTML module. I then created another page with several HTML modules which extended beyond the bottom boundary of my browser window. The two pages rendered with different widths. I then added a number of blank lines to the empty page and sure enough, it then rendered with the same width as the page with several content-filled modules. 

I managed to recreate this behaviour using the standard MinimalExtropy skin under DNN versions is 05.02.02 (73) and 05.05.00 as well as the commercial skin I'm using so I don't think it's specific to the skin.

Any suggestions are appreciated :)

Thanks Rick, you were spot-on - it was indeed just the scroll bars! Can't believe I missed it as I'd been messing around with my layout for ages so I really have no excuse :)