One of the reasons there are multiple "airlines" within an airline, i.e., Piedmont, PSA, etc., is that the operating certificates have been approved and are valuable. I'd guess that when PSA had its ops manual last approved by the FAA, they didn't operate aircraft with availability for rollerboards so they had the one bag rule in there. They just haven't got around to changing it but train their FA's on the restrictions within their certificate.