Everyone does this. People advertise SATA hard disks claiming they're up to 50% faster than ATA100 (the interface is, so it's kinda true, although it's meaningless). Car manufacturers post their acceleration and top speed figures achieved by a professional driver on a closed test circuit with glass-smooth asphalt and banked curves. You'll never get that fast. Fan manufacturers quote free-air CFM ratings knowing full well that in typical useage the fan won't even get close to that kind of airflow. It's just marketing.
What the 6MBps means is that your downloads are capped at 6MBps. You will be able to achieve up to 6MBps download depending on other factors, at the 6MBps point, Comcast will prevent you from going any faster.
If you want guaranteed speed, get a T1/T3 line. Even then, you're limited by the speed of what you're connecting to, backbone traffic, etc, but your line itself will run at X speed. And FYI, T1/T3 lines cost more per month than a mortgage on a respectable house. You get what you pay for, and absolutely nobody at the price point you're looking at will guarantee you 6MBps, so get over it.