with CFD's you are buying and selling the CFDs with the CFD provider. The whole transaction is handle by the same company so the transaction can be settled immediately. How the CFD provider covers the CFD transaction in the physical market is irrelevant.
with shares there is a broker or maybe 2, the ASX and the share registry involved in the trransaction where the ownership registration of an asset must be transferred from the seller to the purchaser. This transfer through the relative parties does take time and that is why we have t3
