You will also see wild claims on cable modem speeds. Your cable provider may have also limited the capabilities of your modem. And, everyone on your particular cable loop will be sharing a 6 MHz video channel that has a total capability of 27 Mbps. As you can see from the above example, if you had just 2 neighbors on the same loop trying to access the network at exactly the same time you are, you would each be limited to 9 Mbps, already under the 10 Mbps theoretical maximum mentioned. But, don't let that bother you too much.
Modems operate in burst mode when they transmit and receive.
However, sharing bandwidth with your neighbors is a very real part of why cable modem performance varies so much. One time you transmit, there may be 20 others trying to access the Internet from the same node at the same instant your are. Thirty seconds later, there may be 50 or 100. But any way you look at it, cable modems have very good performance. You can expect 400 Kbps to 1 Mbps downloads and 120 Kbps to 500 Kbps uploads.