Why is my cable connection so damn fast?

I have a cable modem through comcast. I am now getting in the neighborhood of 700K to 800K per second download speeds. That's kilobytes, not kilobits, which means about 6 megabits a second, doesn't it? How can it be that fast?

I'm paying $45 per month including tax and such, and I thought this only got me 4 megabits per second at most.