Isn't the speed of DSL highly dependent on the details of the actual analog line?
I have an ADSL line, and my contract says "50Mbps", but I rarely actually get over about 4Mbps. As far as I can figure, the reason is because my house is near the limit of distance from the central office... (the phone company actually has a calculator web app thing that lets you plug in your address and get the line length...and then a separate calculator that maps from line-length to expected speed)
[I'm not complaining or anything.... It's a cheap line which I've had for ages, they're very up-front about the caveats of ADSL, and I could easily get fiber or cable (from multiple providers) for a bit more money. So far I just haven't cared enough to change it.]