The permitted Administrative and Sinking Fund levies raised by Owners Corporations (in NSW) are both differential because the units of entitlement for each lot, upon which they’re based, differ from one (lot) to another.
I’m aware of some Owners Corporations who have attempted to raise different levies and to apportion those on some other basis in circumstances such as where some Owners don’t have access to common facilities such as an elevator and do not want to contribute to its maintenance, but so far as I’m aware those types of additional levies are illegal.
If you could provide some additional information on the type of differential levy you’re referring to, for what purpose it’s raised, to which Fund it’s paid, and upon what basis it’s been authorised by the Owners Corporation, then I’m sure you’ll receive some more complete responses.