Re: If its wrong most of the time...
It is trivial to calculate the distance between an exchange and a subscribers premises, but it is almost impossible to determine the length of cable used to connect them, the quality of the copper in that cable, the amount of environmental crosstalk in the cable, the quality of wiring in the subscribers premises, and all the other factors involved in determining your synch speed.
So, given a location, all an ISP can do is calculate a 'best case' and a 'worst case' bounds for your line. Obviously, marketing would prefer that the worst case is as understated as possible, so there will be outliers that have worse conditions even than the 'worst case'.
As an example of this, around Albert Dock in docklands there are some very curious routing of cables. Some locations in that area are extremely close to the exchange, but have 2+ miles of cabling, due to the strange way that the cables are routed around the docks.
Another example is my old man, who lives in the wilds of East Anglia. His line, if you check, says he should get 1MB, just about, because of distance from the exchange. He actually gets 3MB, because the line from the exchange is new, high quality, goes in pretty much a straight line across 7km of fields, and he has excellent in house wiring, including a fitted ADSL filter on the master socket.
With ADSL, you get the physical maximum synch that your line can handle. Switching ADSL ISPs searching for synch speed is inane, the ISPs and BT cannot change the laws of physics. Instead, you should either move, or see if you can improve the situation by improving your internal wiring, or STFU.