So, I’ve just discovered something interesting. In SharePoint, some content should fill the whole of the content area of the master pages; others should have a margin. For example, viewing a document library fills the content section, but Wiki pages need a margin. This is achieved by the ms-bodyareaframe style in the master page, which supplies the padding for the content that is contains.
Ah, but how does the content that doesn’t need a margin get displayed? Well, it supplies an override to ms-bodyareaframe to set the padding to zero – and that is why you should make sure you use the ms-bodyareaframe style!
I found this when working off Heather Solomon’s Minimal Master pages, which don’t include this style.