When you are wrapping up Primary, students are 100% subject to the needs of the Navy for that given week. Typically, class rank dictates who gets to choose first for assignments that they qualify for. However, I can recall a class of three or four students that all had tailhook grades and only one jet slot was on the table that week - the rest went helos or maritime. On the contrary, if you are the only one with tailhook grades and there is only one spot, your dreams of being a helo pilot are over (or imagine if that one spot was E2/C2 and you wanted to fly fighters your whole life). Same system is used USMC... shoot there is the added wicket of TBS, where at one time they utilized a quality spread across combat/non-combat MOSs (not sure if that is still the case).

90% of what you end up flying in the Fleet is timing. It all comes down to how many slots are available when it is your turn to choose. The weeks before and after you graduate Primary may have had all jets, but none your week - makes that week of bad weather, or the one you missed for a head cold suck even worse! This happens again at the end of Advanced when you find out what gray jet you fly, and once again at the end of the FRS when squadron assignment comes into play - particularly so when PRI-A / Japan is in play.

Timing, timing, timing... all you can do is perform your best at each step along the way and be as much in the driver's seat as possible.