If it has an altimeter with an electronic subscale the subscale decrements in discrete steps, rather than continuously as did older clockwork altimeters.(e.g. as in the Vickers FunBus - 3 clicks per millibar if I recall correctly - or alternatively in precise 0.01 inch Hg intervals as it had both options).
Given the relatively coarse assessment of touchdown zone QFE at most military bases (usually derived from standard correction applied to a value obtained from the Met Office up to an hour aga, the error displayed at the threshold using QFE may well put an RVSM-compliant altimeter out of limits if mB are used, whereas with precise inch Hg values, the greater precision may put it inside limits as you have 1, not 3 subscale alternatives.
The solution is to have instantaneous exact QNH inch Hg values available, plus accurately surveyed parking position elevations. So that you can check whether or not your altimeter is really within limits. But that would cost monry, so just fudge it and hope - as with most military compliance with current rest-of-the-world requirements.