I can tell you it's that way by design, notice that if there is content in the sidebar, then the footer goes from 4 column to 3 column to leave a "space" for the sidebar to comedown and overlap if needed.
I went a few rounds about this myself before I realized what was going on design wise.
I never did find a way to "fix" it, but I'd assume you'd not only want to make the sidebar push the footer down, but also restore all 4 columns in the footer as well so it doesn't appear shifted to the left....