You are right, Group III has slightly better solubility than PAO (both still require carrier oils for additives with better solubility), and you can build an excellent oil using Group III. That said, PAO has better cold temperature performance as well as oxidation resistance, doesn't require PPD's to achieve that cold temperature performance and is a more expensive base oil, so if it is being used, you know the blender has made the decision to spend more on that product to formulate it. In my mind, that's choosing performance over cost, though it should be noted that if two products with the same approvals are blended differently, one with Group III one with PAO, the end user is not going to be able to discern the difference and they should perform similarly in service, relative to those approvals. A Mobil 1 oil having better CCS and MRV values than its Castrol competitor while both pass the requirements stipulated by J300 for the grade, points to the blender making choices about other aspects of the product, like choosing to use PAO instead of a thinner base and more VII for example.
Personally, if I can buy two oils for the same price and one is primarily PAO based and the other Group III, I'm going to go for the oil the blender spent more on formulating.