Perhaps some of those posts are not filled with a steely-eyed two-winged warrior. Perhaps some of those posts haven't been actually filled for years, either by aircrew or indeed by anyone.
Things change. My own Branch went through a big re-structuring and shone a bright light in the corners of the manning plot and did some mature thinking. Some posts were not a priority - not for the Branch or even for the RAF as a whole. Things around the posts had changed and things hadn't caught up.
I'm not saying this is the case here, but it will probably be part of it. It is always interesting, for those who can, to access the spreadsheet on the manning website and sort it by Branch. There are some strange and wonderful posts out there that don't get filled or have little, if anything, to do with the Branch.