I'm not very familiar with Artisteer and I don't know why it's rendering that particular paragraph, but I can tell you that the reason it is breaking on the contact page and not on the others is that the paragraph "art-page-footer" is being added inside of the containing div "art-sheet" on the contact page.
The difference is that on the other pages it is being added outside and below the containing div "art-sheet", so the white background is not being pulled down to wrap it.
If you're not using the footer paragraph, I suggest adding the following CSS to your skin.
Either way, it still might be a good idea to look into why the paragraph is being added to the DOM in the first place.
Hope this helps,