PLA Organization & Structure discussion

as we all know PLA is divided into CAT A and CAT B armies. the consensus used to be that CAT A has better of everything. but that doesnt seem to be the case for Nanking military region. cuz i hear a lot about the 31st GA, which is a CAT B unit, but since they are right across from Taiwan they get the best of everything. and i also read somewhere that they have really hard training as well compared to other GA's. the next most well-known GA in Nanking MR is the 12th...which is also a CAT B. and the least well-known seem to be the 10th which is a CAT A. so why dont they simply change 31st into CAT A if they like it so much? is this bureaucracy at work or are they trying to avoid somehow angering Taiwan? i dont know much about the rest of the PLA but Nanking MR is one place where CAT A and CAT B variation doesnt really hold out.