I raised a similar question regarding b/b speeds a couple of weeks ago.
In the main, I would perhaps suspect that the ISP's are selling the service on higher speeds, hence the usual 'upto' disclaimers. Oftel have had discussions about this, and in the very near future, a more realistic figure will have to be provided in adverts and sales literature.
On a regular basis, I use a number of various speed-test checkers, so as to see if they are in agreement with each other. Usually they are about the same, but on odd occasions you get the very strange reading, but then again various speed test do warn you about accuracy.
Going a little further, I am having various problems with my ISP. One particular problem is the low speeds and frequent disconnections. My near neighbour using the same ISP,via same exchange and cable length can achieve far better results. An example the other day, I was obtaining 2.3Mb and disconnections, they were getting 5.8Mb and no disconnections. The ISP is still working on the problem!.