Fixed Variables

Kai has previously blogged on this, the practice of charging/paying a fixed amount each month for a variable bill. How does this make sense? Who benefits? Do the companies hope that eventually you'll start over paying and forget about it? I suppose it might be argued that the customer benefits as their bill is buffered. I was pretty shocked at my latest BT bill which stated that unless I contacted them they would more than double my monthly payment. How did they get it so wrong in the first place? My bill is very little over my fixed monthly line rental. I called and asked why I couldn't just pay the actual bill each month, well I can!