Never did get a reasonable explanation why the military chose to revert to using QFE, especially as the change was very poorly notified, whereas the change to QNH procedures was well planned and executed, with the excellent Dave Harrison visiting most units to brief them. I WAS told it was because an 'elderly gentleman' (senior officer to those unfamiliar) cocked it up one day on a check ride thinking he was on QFE and trying to descend to zero on his altimeter, but that was only a rumour.
IIRC it was a large lobby of Transport pilots who pressed for the introduction of QNH, that was ignored by a silent majority... until it was introduced and then a vociferous bunch of FJ pilots stood up (probably inc the senior gent mentioned) and demanded a reversal on the grounds of flight safety.
The RAF introduced the change without reference to the RN, pushing ahead with an ammendment to Military Flying Regulations... which as you can imagine didn't go down well.
IMHO the snag was a making an attempt to switch from one pressure setting with the total exclusion of the other that casued many snags. The effect of this Cock-up has had a lingering effect, even mention QNH anywhere near STC these days and they'll cry foul citing this badly managed/considered event
back to the subject of the original post... one of the problems of RPS is that its a forecast lowest pressure acroos the region... some of these are large, so for example when there's a massive Low in one corner the difference between RPS and QNH in the opposite corner (and indeed between adjacent RPSs) can be significant.