Service selection is very competitive among STA-21 Core options as you are only competing against other STA-21 Core options. I think last year they only selected two pilots and maybe six NFO's, out of about 40 Core options. I know of two people with competitive grades/aptitude/etc that didn't get their first choice. If you want to be a pilot, put in for the pilot option. There are no guarantees with the Core option when it comes to service selection*, no matter how good your grades/aptitude/etc.
*unless you want to be a nuke or a swo
I'm not entirely sure your information is accurate. It's my understanding that we (Core Options) service select right alongside the other MIDN. I wanna say its in the instruction.