See my reply to your other post that should fix all your issues. As far as your current Q: block elements are 100% width by default. so it’s actually being centered you just cant see it since the auto end up being calculated to 0.
Ok. Ryan brought up a good point the default SETTING = AUTO. Which does differ slightly from actually setting width:100% by the fact that padding and border will not be added to the calculated width. So I will rephrase , a block element with width:auto ( no width set) will by default extend from left edge f its parent to the right edge of its parent, and thus leave an "auto margin-left and auto margin right of 0 each.