Another Beef With Colloquialism

You know the saying "below par" as in "his work is below par" or "that performance was below par"? Well, in golf, if you're shooting below par, than you're doing well. It's a good thing to be below par. So how does saying someone or something is below par equate to it being a poor performance?

We talk about people being "on par" as in they are on track or doing the right thing, so why isn't someone who is below par doing better than someone who is just on par? It just doesn't make sense to me.