Your right on that gerald and there is also tcp/ip over head that is factored in as well that will be the deciding factor on what your speeds are. It's a small overhead, not near as much as line noise or distance, those are usually what slow it down the most.
Headband, that's just you and likely your area getting that speed. My connection is 3mbps/256kbps, I usually get 390KB/s, however that is what it reported, with out the overhead, it's around 375KB/s.
Either your area is over used or your having problems elsewhere like a weak signal stregnth or a few tweaks that could help. Like too many splitters or etc or something causing packet loss. I am guessing your area is over used. Cable is shared with your neighbors, so it's like your on the same network as them. If several of them are downloading large files or playing games etc etc etc, this can impact what speed you have. THis is likely your cause of slow speeds, as it is possible to reach the full potential. I have cable and get full speeds, however this is because not too many people have oversubscribed their service. However some isps will take out their faults on customers, rather than making sure there is enough bandwidth for all users, they will sometimes put caps on the line that slow your connection if reached a limmit or like comcast and cut your connection if over used. Which is completely unfair.
However If i were you I would contact your cable provider and ask them what they think the problem might be, like a new splitter, boost signal stregnth or something else where you might be getting loss.
DSL has a set speed that it will run at, however a few things that effect it.
Tho there are few that cap anymore, mainly becuase people will stop using them and find some one else. it's bad for business when the company can't handle the load and take it out on the customers. Which is good when there is competition that wont do the same thing.