Not to be contentious, but that isn't the case. They do a decent job of summing up your overall ability, to heavily include performance in airmanship programs. If it were grades alone I wouldn't have been selected.
OP, I would say try to go ENJJPT in all cases unless you are dead set on going to AMC. All 38 drops cater to the needs of ACC, AFSOC, and AMC, in that order (I forget where GSC goes in that order). You get a different syllabus with more exposure to different types of flying which is if nothing else, quite enjoyable and could possibly help you make a more educated decision when filling out a dream sheet. Also, given the reduced frequency of drops here, if you're a strong swimmer they may give more consideration to getting you what you want, not a guarantee, but a greater likelihood. I also wouldn't be intimidated by the "stronger competition", unless you have 0 prior aviation experience. Feel free to send me a pm if you have further ENJJPT/38 related questions.