Jon's question is valid, but Disk Utility has long had a reputation as a very weak disk repair utility. Sometimes running DU multiple times will eventually effect a repair that was not fixed on the first pass, which may have happened in your case. Disk Utility was improved in Leopard and Snow Leopard but on a scale of 1 to 10, where 10 is top of the heap, it is probably about a 2 or 3 so common wisdom says you should have one or more of the third party disk utilities.

Diskwarrior is generally considered to be the best volume repair utility, but TechTool Pro and Drive Genius have both been known to repair a volume after Diskwarrior had failed and both TTP and DG both have feature sets beyond what is available in Diskwarrior. For what it is worth, the last two times I have taken a Mac to the Genius Desk, their first set of drive tests were performed with Drive Genius. Personally I think everyone should have a bootable external drive with one or more of these utilities installed － just in case. I have TTP and DG on mine.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein