In the specific case of rwy 23 @ GLA, it's my understanding - never having worked there - that the original levels flown whilst providing terrain clearance from the ridge caused certain Ac types GPWS to go off because of rate of closure to the ground albeit stopping descent at a terrain-safe level.
Therefore a higher level to accomodate one A/c type became 'standard' for everyone. Without having an RVA chart to hand, I suspect the
actual RVA level is lower than 3000ft, but that is the practical level used until late into the ILS closure to prevent unwanted GPWS-induced go-arounds.
I'm sure I'll be corrected by someone in the 'know' if I've got this wrong
Tori