Unfortunately, for someone applying for a commission in the RAF, it does matter - regardless of how tosh we all think it is. I believe OASC are being pretty damned hard over on the figures for entry - given the feedback I've seen elsewhere from recent applicants.
BMI isn't that far out for people of an average build - those of us at the anthropomorphic ends of the scale certainly don't fit on it. BUPA - just because you don't think you are overweight does not equate to a wholly objective assessment does it? Perhaps you are.