What available bandwidth should I have?

Our nominal bandwidth (or basically what we're paying for) is 2.0 mbps or 2048 Kbps. This is equivalent to a download rate of 256 KB/s.
In fact, our download rate here maxes out at about 218 KB/s or 1744 Kbps or 1.704 Mbps.
A bandwidth tool I've been using recently confirms that our bandwidth maxes out around 1.55 Mbps or 1587 Kbps.
So, why aren't we getting 2 Mbps?
I had a long discussion with three different people in Eircom Broadband (our ISP) today and got absolutely nowhere.
The first guy told me he was going to increase the interleaving to increase the speed. A second guy said this was rubbish. However, he said we should be getting something above about 1750 Kbps. He was about to start into some tests when I made an error and lost the connection with this guy.
Called again and got another guy who got me to do some speed tests using the Eircom test (which I thought only worked in Windows with IE -- however, it does seem to work in Linux with Opera).
I did a lot of tests using this meter and got typically around 1620-1680 Kbps. The guy then told me that 2 Mbps only guarantees more than 1600 Kbps so they were in compliance with this.
So after about an hour I learnt that not only is there nothing they can do but they don't have to as 2 Mbps actually can mean as little as 1.56 Mbps.