Wireflyer: Please note that twin A/P autolands ie. Fail Operational allow the use of both Conf3 and FULL for autolands on all the variants A319,A320,A321
My guess would be that since the 319 and 321 are much newer variants then Airbus simply decided to not `revisit` the certification process and leave this anomaly in place.
Remember the A320 was certificated in 1982!