There is provision in the NSW Strata Schemes Management Act which may legitimise the practice adopted by your OC. Of course that would depend on agreements being drawn up and being acceptable to both owners and the OC. I’ve never heard of the term differential levy. See below:
111 Can an owners corporation provide amenities and services to a lot?
An owners corporation may enter into an agreement with an owner or occupier of a lot for the provision of amenities or services by it to the lot or to the owner or occupier of the lot.