AFPRB is independent from the MoD; it is not independent of government.
The case for service pay used to be laid-out by the service chiefs. The Treasury effectively neutered them years ago by awarding the funding for the coming year many months before the ARPRB produced its findings. If the AFPRB recommendation is ever above the Treasury prediction/cap/limit (aka Comprehensive Spending Review) then the missing balance has to be found from the MoDs existing budget - effectively triggering further unplanned cuts. The net effect is that the service chiefs are rather reluctant to support any pay rise that could be above the amount already stipulated by the Treasury Spending Review.
As such, the scales are tipped away from any effective pay rise. The only variation is the AFPRB excuse-of-the-year to magically arrive at the same figure stipulated by the government before the pay review board even starts. The only possible change is when politicians feel the need to increase military pay. Amazingly the AFPRB will somehow arrive at this exact same politically driven percentage. About the only exception to this was the X-factor increase a few years back that neither the Treasury or MoD were expecting. The bun-fight lasted well into the new financial year and the then Chair of the AFPRB lost his seat.