Check the Cablecom "small prints" in the contract. It says that if you have heavy traffic during working hours, Cablecom is allowed to reduce your bandwidth. You are right, the cable is a shared medium and if one users sucks all the bandwidth it slows the other users.
Cablecom has good products, but it's fine as slong as everything goes well. Trouble will start as soon as you have a problem. The customer and support service of Cablecom are well known for being bad.