I believe Joel Spolsky coined those terms in an essay about hiring. The reference is to American grading - letter grades are A (best), B, C, D and F.
He didn't, that I recall, define where the difference between an A and a B player was, so it's kind of a difficult metaphor to apply. It's really got nothing to do with any American sport that I know of - those terms would be more like "bench player", "starter" or "utility player".