If your phone bill is figured on the first,due on the 20th and you pay on the 20th,but did not cancel service until the 15th, you will owe for the service from the 1st until the 15th.
Even though it is easier to understand with the phone bill, the theory is the same.
You were given a courtesy credit for it because it doesn't seem to make much sense to be told you owe a certain amount and not have it be correct.
(I think I got this right