They navy/marines drops are based entirely on your MASS score (its basically every sortie, test, anything graded you ever did), and its a moving average based on the people the month prior that dropped. The marines have a minimum baseline score you have to get to get jets usually. If everyone the month prior to you sucked it drags the scores down you could get tailhook or it could go the other way and everyone was shithot and did really well you could possibly not get it even if you did really well.
That's the quick and dirty as it was explained to me by my DoN classmates at Vance. I think BolterKing is a navy guy, he might have more detailed info for you.