why increase bill IF I signed 2 year contract

I don't understand why the bill can be raised after just signing another 2 year agreement that I can't get out of without penalty YET comcast/xfinity can raise prices. Other companies do NOT do that, they give you a price and possibly adjust service up or down and that is your price for that time period. IF price goes up, then why am I held to 2 years.  the person NEVER told me it could go up or for what reasons. a rip off