Why are my parents mad about my grade?

In school, all of my academic grades are A's. In PE however, I have a D-. The reason for this is because at my school we get graded for how fast we run our miles. I am not a fast runner but I am also not fat at all. My mom is mad at me for this and says that I am not trying, but I am. My dad says that they shouldn't grade me on how fast I run and he wants to talk to my PE teacher.