Game Drawn For Insufficient Material

Would someone plesase explain this rule to me as applied in my last game.  My opponent had used up his time.  He had a rook and king against my king. I can't understand why he gets a draw, since he had sufficient material to mate me if he hadn't used all his time.

I think this is the game he is talking about:

In the final position, white ran out of time. However, black can't win by any sequence of moves (he only has a bare king), so he doesn't get a win on time. The game is drawn.

To rephrase xbigboy's post, YOU had insufficient material to mate, and therefore you can't win on time.

Thank you, xbigboy.  I thought that if you ran out of time you lost.  Obviously you are saying there is this excepion, which I wasn't aware of.  It makes sense, but surprises me.