Interest on personal loan calculated incorrectly

So how does this work: FNB personal loan is deducted on the 13th of December (normal due date the last day of the month) and the interest calculation remains exactly the same as previous months?

I queried it with FNB and was met with this response: Please be advised that the debit order for December was processed early due to the December public holidays and customers receiving their salaries early, you pay the interest for the whole month

Surely if your interest is calculated on outstanding balance, there ought to be a difference in the interest levied? Or am I wrong in seeing this as a blatant theft of funds, them profiting twice on my funds paid over to them?