Grades vs. Test Scores.

I've just read How Much Do You Value Test Scores by Mr.Fisch. This article really hit home with me. It was about test scores, and their relevency. I am sortof split on this topic. I start defending one side, but end up switching to the other, and vice versa. I think that mostly I side with pro-tests. I usually do very well on tests, but I recive very bad grades. I don't think I get low grades because I'm not smart, I just don't do my homework and that's usually the biggest problem. I see myself as a pretty smart person usually and part of how I assess that is through major test scores like CSAP and MAP and basically the acronynm tests. I usually score a lot higher on those than I do in my classes. My question is do a persons grades in a class realy reflect their compency and understanding of the material?