Also, AP and cover modify *save rolls* not the saves themselves.
So 2+ out of cover hit by AP-1 still have a 2+ save, but you’re at -1 to the dice so they now roll 1, 1, 2, 3, 4, 5 (I.e. a natural 2 is now a 1 so fails).
Getting the benefit of cover adds +1 to the roll which counteracts the -1 so a natural 2 passes again.
The no benefit vs AP0 for 3+ or better rule is mainly there so that 3+ saves don’t double in effectiveness vs small arms in cover (which was a bit much in previous editions).
It’s generally irrelevant for 2+ saves, outside of some nice situations where they get -1 save from other means.
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