Looks like Airbus don't want an epe less than .28 uless GPS is valid and used.
May well have something to do with the implamentation of RNP when the system was designed. As RNP approaches at one time also specified the nav mode eg. RNAV(GNSS)
B-RNAV = RNP 5
P-RNAV = RNP 1
Default Approach value on our brazilian/honeywell equipment is RNP.3
As RNP is stll evolving I woudn't spend too much time worrying as by the time you've figured it out the game may have changed.