I would suggest understanding the base technology of the services you are considering.
Cable for example is a shared platform. You may share 40meg of capacity with 100 other people and be individually limited to 5meg. Depending on the users, you may or may not not ever have the network congested so you will receive less then peak value, but that doesn't mean it won't happen.
You could be unlucky enough to end up on a node with dozens of heavy torrent users.
The up-to is probably generally acceptable because 95% of consumers don't understand the underlying technology and probably are not as concerned about if they get 5meg all the time, or if sometimes they only get 3meg.
The speed they are giving probably refers to the physical cap on your access path, and the up to is to cover them off when the load simply get overwhelmed. If they are good company, they either have have tons of capacity, or they fire customers who use excessive quantities of capacity, most likely a mix of both.
Mr. Cole's Axiom: The sum of the intelligence on the planet is a constant; the population is growing.