HOW CAN A SETTLEMENT AMOUNT CHANGE WITHIN THE GIVEN PERIOD???

I genuinely do not know what is going on. I emailed for a settlement letter and got a response the 11th June. I paid the complete amount on the 15th June. When i pulled my statement now it shows there's still an amount outstanding? How did that happen when the settlement amount was given to me!!!! And i paid in time, within the 7days the settlement amount is valid for!!!! I refuse to pay more if this is the amount given to me, its pathetic! Sort this out and send my paid up letter!