SL,
It's not rocket science (actually rocket science is pretty straightforward!)....
Let's take an analogy you will be familiar with. In your last year in the military you won't get the full leave entitlement (38 days), but a proportion thereof. In your case, leaving in Jun 2014, for the period Apr-Jun 2014 you will probably get (without knowing exactly when in June you will leave) about 30% of this figure, some 12 days annual leave.
Your FIRST, and only your FIRST, pension increase works this way. In Apr 15 you will receive a proportion of the annual rise in pension, directly related to the proportion of the year you have received that pension. So, in your case, in your FIRST year, i.e Apr 15, you will receive about 83% of the total pension increase (which will be the CPI figure for Sep 14). So if the CPI is say 2.5%, you will recieve about a 2.1% increase. In all subsequent years you will receive the FULL increase, i.e. the Sept CPI figure for the preceeding year.
I hope that makes sense - it does to me, but then I wrote it!!
Yes - leave just after Apr and you get a bigger rise in your pension the following year, but for every month you stay in past Apr you are probably earning at least twice what you would draw in terms of your pension if you were out......