'Back in the Day', I thought that a 'Spec Rec' normally implied that one should be promoted at the next available opportunity....
Clearly not though - it took me 3 consecutive SRs before I was promoted to Sqn Ldr Spec Aircrew
It's called Assessment Creep and it has nothing to do with anything other than people not knowing how the system works and/or not wanting to be honest when writing assessments. If you and all your peers were Spec Rec, then who is the best and truly worthy of promotion?
I was lucky enough to observe on a Promotion Board, then to serve on one. It changed my view and writing of annual assessments. Creep just saw everyone getting Spec Recs (or 6 in more modern parlance) with no substance in the narratives. Having to explain to my troops that '5's and Highs' would get you promoted took a lot of effort, until they saw it themselves.