If you employ your PA, the direct payment rate must cover not only their wage, but also other costs needed for their employment.
These costs form part of the amount the council will fund for your Direct Payment and so the council have a set rate of £12 an hour as the amount you can pay your PA.
If you are using a self-employed PA, you can pay them the full direct payment amount as they need to pay their own tax and arrange other things such as holiday and redundancy.