Graded on the curve.
They were both graded against their own past performances in the public eye.
Biden is known as a passionate, hot-tempered, hammer & tongs debater who's not afraid to go out on a limb. He reined that in, and people who expected to see some of that knocked him down a grade for being unexciting. Also, I suspect, for being old and male.
Palin was expected to fall on her face. She didn't, which was enough to give her an A to start. She got high marks for her charm as well. She also gets novelty points. People recognized the absence of substance behind her talking points, however. So she got knocked down a grade for that.