Actually there are a number of reasons why your service may be slower than it was. First and foremost is the number of subscribers in your area. It is true that the bandwidth is shared among all the people in the area. The more people on it, the slower it can be.
Another cause for the slow down can be headend equipment in the cable companys head end. The more Fiber nodes connected to the UBR router, the more bandwidth trying to get through the same pipe. A second or third UBR in the headend can alleviate crowding, but the cost and space required for it may not be available.
Another cause can be ingress on the cable lines from outside frequencies. Cracks and damages to the physical cable can allow off air frequency into the shielded cable lines causing interference with the forward and return signals to your cable modem. Usually this can be identified by a visual check of your television set. Any off air broadcast channels will have a ghost or other such motion in the background caused by the ingress. If there are no anomolies on your local abraodcast channels, then there is most likely no ingress.
Yet another cause can be water in the cable lines. Could be in your drop line to your house. Could be in the main lines outside. A check of your signal level with a meter can tell you if there is water since it will attenuate the signal.
Loose connections can cause problems as well. Also splitting your signal to feed multiple outlets and/or cable modems can attenuate the signal as well.
It is true that what the customer deems acceptable download speeds and what the company will work on are two different things. As a Network Technician responsible for the outside lines for the cable company where I live I can attest to that. I have @home service here in the Portland OR area and I have seen my download speeds vary significantly.
Have you run the Bandwidth speed test at
http://webservices.cnet.com/bandwidth/I just ran it and had 2008 Kbps. I ran it again and had 435 Kbps. Ran it again and had 635 kbps.
So as you can see, bandwidth varies. But the cable company is correct that there is no guarantee of download speeds. Check your contract. It may be different in your area.
Ed