They use a 2-cycle billing method, which is EXACTLY why you THOUGHT you had it paid off, when in fact, they were going to make money off of your previous month's balance. Go back and look at your terms, and I'll bet you'll see something about "2-cycle" billing or something to that effect.
I didn't realize that was the case until I bought my Dell computer. If I opened an account with Dell, I'd save some percentage, whatever...I forget. Anyway, I bought the computer using them, that's when I got my first taste of 2-cycle billing. I had a 1200 balance one month, sent them a check for 1200 and the next month I had like a $5 and change balance. WTF?? I went back and read the fine print and there it was..2-cycle billing. Dammit..so I sent them 25 bucks and made them send me about 3 or 4 statements with a credit on it before they refunded me the credit via a check. Then they had to send me another statement the next month to show my balance was zero. So I got a little bit of satisfaction knowing that they had to carry that credit forward and send me statements costing them a little bit of postage for a while.