It should be borne in mind that speedtest.net (and other sites of that nature) do not by any stretch of the imagination provide results that can be considered scientific. Nor can their results really be used as evidence against your ISP.
The problem is that there are so many points at which contention, bottlenecks and throttling can occur between you and the speed test website that even two tests conducted one after another can provide vastly different results.
As I've said before on this site, a more realistic test is to try a large download from a well connected CDN (e.g. Akamai). Or even better, a large download from a server hosted on your ISPs own network.
Just remember. *DSL speeds are NOT guaranteed and will always be subject to rate limiting, packet shaping and all sorts of other wizardry (even mroe so for residential customers). You get what you pay for in this life. Invest in a leased line if you want 1:1 contention and "guaranteed" speeds.