It’s TV Licencing regulations.
You are building credit for the next year's licence, effectively spreading the cost and ensuring you always have 6 months worth of licence paid in advance, as required by law.
It’s not unlike energy companies which like customers to be in credit.
You aren’t being charged any extra for what is effectively a credit arrangement.
There is an alternative to pay quarterly where the annual charge is divided by four but there is an additional charge for that.