Does tax deduct from one-off payments? When?

When I make a one-off payment to a student, tax isn't deducted right away. Does this happen on the payment day I've selected?I have enabled the tax module but tax doesn't seem to be coming out straight away. It seems odd that the one-off payment is added straight away to their balance, and the tax is deducted later on the "payment date".

Scott Passmore