Interest Charged after balance is paid in full?

Hi,

My dad recently paid his MBNA credit card in full via faster payment 3 days before the payment was due. 3 days after he paid in full a payment was still taken out of his account (he was paying by DD each month) and was then credited the amount minus interest.

I’m a bit confused by this as he had a statement saying what the final balance was but nowhere did it state that they was interest to be paid on top.

Can anyone shed some clarity on this?

Thanks