CBAT is only used as an indicator of your likely success in the early stages of flying training - for Pilots that is ground school and Elementary Flying Training (EFT), and for WSO/WSOp that is for ground school and the foundation stages of training. CBAT is one of a few things that they look at for OASC - the others are the interview (just shy of 30 mins), the hangar exercises, the group discussions and the medical.
Streaming for Pilots takes place after EFT and they look at their training folders, their preferences and where the Service needs them to go. That Service need is always the biggest driver as to their destiny. As a sausage machine that keeps flowing, they have to dispose each set of trainees at each streaming board and it is not possible to keep parking folks capable of going to their preference because something might come up next time around. If you did that you would end up with a right mess…