Who said it was? Let me rephrase this,

The cable modem is limited to 45 Mbps.
This does not mean you will get anywhere near this.
They would have to dedicate a T3 to you if you were going to get this much.
It's not up to you how much bandwidth you have, it's up to the cable company.
Go complain to them about it. And if you're saying you get 3-10 Mbps on average, what's the complaint there? Are you trying to run a server of some kind? Every cable company's ToS I've read does not allow this. End.