I had this problem just before I called in to cancel. It was actually the reason I called in. When I talked to front line tech, they actually asked me to multiply the number of hours in a day versus the speed of my cable and "proved" that it was "physically possible to upload Nth number of gigabytes in a day"
I told them flat out that even though it may have been possible, I didn't transfer that much data in one day (it was aroudn 65gb) and that I was NOT paying overage. I cancelled my account on the spot, told them to come pick up their cable modem and get out of my life.
Just call in, and raise hell. Refuse to pay. Laywer up.