Sorry, I deal with this every year. The proficient grade level score means an ADVANCED grade level score while a basic grade level "mastery" score is in another category. Grade level mastery would include many more than this (though for DC, I imagine that STILL falls far below what it should). As I said, there are many ways of looking at it.
Then again, since one half of the population is to the left of the bell curve, why should we be surprised?
I concede that I was not taking into account the myriad ways the public school system has of pigeon-holing and nuancing.
Nonetheless, based on my dealings with the system, I suspect basic grade level "mastery" to be not even worth noting. One of the reasons I say that, is that 8th grade reading and math standards are in the gutter to begin with. Basic mastery of them means very little.
When I completed the 9th grade, I had easily mastered what are considered 12th grade standards now. With perhaps the exception of advanced math. No, I take that back. What was shown to me 6 years ago as advanced algebra at the local high school (well ranked, I should add), compared with my 9th grade algebra one.
In 1967, when I finished the 9th grade, I could have sleep walked through a contemporary senior year. Point being, that even "proficient" levels are pretty crappy.