I don't know what the situation is with cable, but with ADSL the effective throughput is dependent upon how many others in the same contention group are active at any one time. In simple terms, if no one else is active you should get the full speed (assuming no other limiting factors). If others are active the throughput will be degraded. In practice, very few people use 100% of the bandwidth for anything other than relatively short bursts.
I'm with a provider that has a 30 to 1 ratio, but I've never noticed anything other than virtually full speed - every time I've done a speed test it has been around the speed I've paid for.