the username is from 2006 or maybe even earlier, dont remember.

the calculation of the your price at the end of the rental is purely IT based.
either you returned the car late or the tariff was incorrect when you got the keys to the car and you did not mention it on the rental contract.

my guess was, that a weekend got over-extended. if you say, it was not returned late, then something at the beginning must have been wrong.

or do you believe someone in Sixt will during the rental manually change your tariff code?