Remember kbps is kilobits per second, and that's how modems are commonly rated. The Optimal Download Speed columns contains idealized numbers. There are line conditions and variable protocol overhead that I cannot estimate for everyone, so I used the ideal numbers to represent the relative performance differences. The Real-World Download Speed is my best guess at what I think it would really be. As always, your mileage may vary, but the e-mails I have received tell me I'm pretty accurate here.
Optimal cable is what the current hardware is set up to be able to do as a theoretical maximum.
I also included 10 mbit LAN for a comparison figure, since regular readers of this site probably have one at home. When cable modems first were being rolled out, the cable provider just set everything for maximum speed and let it go. What happened is some people got incredible performance at 4 in the morning, and complained that they couldn't get that speed all the time. You may not peak as high, but it ensures that they will have enough resources to keep everyone at reasonable performance levels because they know what maximum to estimate for. It is also critically important to realize, when judging the speed of a high-speed access solution, that you are now a peer with many web servers.