You didn't give enough details.
Case 1) You have a hidden fee for an unrelated issue that we could never predict since we don't know anything about your situation.
Case 2) You carried a balance and were paying interest. Interest accumulates each day. Between the time the bill was printed and the time they received your payment, more interest had accumulated. Thus, you didn't pay it off in full. Take this crude example as a lesson:
Suppose you owe me $1000 today and I charge $1/day in interest if you owe me any money (regardless of your account amount). If you pay me $1000 today, you'll reach $0 and be there for as long as you never borrow money from me again. But if you pay $1000 in a week, your account is $1000 + $1*7 - $1000 = $7. you didn't pay me off in full. You should have paid me $1007 even though the bill only said $1000. I won't bill you for another month, so when you reach that next bill you'll owe $30 of interest because you didn't pay me in full on the day I wrote the bill. Forgive me for using a slow method of mailing you the bill, to guarantee you won't pay it off in full. :evil:
The solutions: (A) call for the payoff amount and pay by phone that day so no more interest is charged, (B) pay more than you owe and have them credit you back any excess.
Case 3) You have two-cycle billing. This means that if you owed interest 2 bills ago, you are still charged interest on that even though it was already paid off in full last month. 2-cycle billing is a devious and unethical practice. Luckilly it is becoming less common. The solution to this is the same as above: call them and pay more than you owe.
To avoid this in the future, pay it off in full every month no matter what. Then you'll never have to worry and never have to pay more for all these fees/interest.