I used to be a cable tech, I bet they did not know you had two computers, what I bet they knew was that you had two different mac addresses.
On DOCSIS modems, they register the MAC addresses of any device that hooks up to the cable modem, even if it is no longer hooked up - even if the cable modem is unplugged - it stays in memory and the Cable company can see it anytime they Poll.
If at one time, you had the computer hooked up directly to the cable modem, that is one address they recorded.
If you then added the router, they recorded that one as well. I do not believe they would detect a computer behind the router itself. I know the modem itself can not.
99.9% of the people on the phone do not understand most of the stuff on the screen. I have talked to 100s of them, and most do not know what they are doing.
I have seen the internal software cable modem providers user to track their system. While they do track your MAC address, amount of data transfered, DB level of your line, packets resent, and other stuff - they make no effort to determine the number of computers. I do not even believe there is any option for that.
On many cable modems you can access the internal DOCSIS screen by going to 192.168.100.1 in the browser itself.
You will see the list of MAC addresses I am talking about.
The people on the phone always jump to conclusions. By the way, I have seen many routers fixed just by doing a flash upgrade. This has happened on three of the Linksys routers I have used or helped install.
The Linksys router will also let you spoof MAC addresses in case the bozos at the cable company have a problem with your MAC address.
Then again, this was for only one company, other companies may vary. The DOCSIS standard is pretty strict though, and I do not recall seeing anything in there for individual computer monitoring off one IP - I could be wrong though - it has been a while since I read it.