This is the major problem with the all or nothing nature of the current
AP system.
It completely messes up game play and game balance in so many ways!
Armour save modifiers are a bit clunky, and although give better proportional results.
They still do not give enough granularity needed by later editions
40k IMO.
The alternative
AP method I suggested is less clunky than ASM in practice.But has not got enough granularity for my liking.
(This is why I prefer opposed stats of
AV vs
AP on a chart

.)