Martin,
Just one small factor to add to the mix. In 1938 the Air Defence Cadet Corps (ADCC) was set up by Air Commodore John Chamie RAF. This would later become the Air Training Corps and later still, the Air Cadets. Apparently thousands of cadets joined in the first year, and if they were allocated numbers from the overall RAF pool of numbers (I don't know if this was the case), then that would seriously distort the adult recruit numbers based on your calculation.