Like he wasn't even trying. Apparently that's the only time he has ever wagered that low on Final Jeopardy.
Maybe he was just banking on her missing it. He probably knew exactly how much she would wager, and if she got it right, he had no chance of winning anyways. So maybe he figured his best shot was to risk just enough to keep the third place guy from somehow beating him (in case he got it right - which he did), and then just fingers crossed that maybe the lady in the lead misses that last question.