Perhaps my explanation and example weren't great? Here's a very simplified version of the same issue:
Why does the orange box just keep going right, even when Y > 400?
Edit: Solved by changing Y to X, though I still have absolutely no idea why that worked. I just happened to try it because I'd already tried everything else I could think of <img src="smileys/smiley5.gif" border="0" align="middle" />