What should grades reflect?

My thought is that grades should reflect the mastery a student has of a given subject. Is this reasonable?

Ann Althouse found an reference that Standardized testing has embarrassed teachers into facing the fact that they've been grading kids for compliance and pleasing.

Ann links to A’s for Good Behavior which starts off with:

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A few years ago, teachers at Ellis Middle School in Austin, Minn., might have said that their top students were easy to identify: they completed their homework and handed it in on time; were rarely tardy; sat in the front of the class; wrote legibly; and jumped at the chance to do extra-credit assignments.
But after poring over four years of data comparing semester grades with end-of-the-year test scores on state subject exams, the teachers at Ellis began to question whether they really knew who the smartest students were.
About 10 percent of the students who earned A’s and B’s in school stumbled during end-of-the-year exams. By contrast, about 10 percent of students who scraped along with C’s, D’s and even F’s — students who turned in homework late, never raised their hands and generally seemed turned off by school — did better than their eager-to-please B+ classmates.
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While there is value in learning how to get along with people, a math grade should give the student and parents some idea on how well the student knows math.

A few people attack homeschooling claiming that without professional training parents won't know just how much their children really know. It is clear from the above posts that there are teachers who don't really know how much their students know.