What Milo said.
Speed test (note the space, I'm talking generically here !) need to be taken with a significant pinch of salt.
There are so many factors that could influence the results, including but not limited to :
(a) Your infrastructure (problems on your PC and home/work network)
(b) Your ISP (packet shaping, contention etc.)
(c) The path traffic from your ISP takes to their ISP
(d) Their ISP (packet shaping, contention etc)
(e) Their infrastructure (e.g. temporary problems on testing server)
I have found "Speedtest" to be particularly unreliable and inconsistent. "thinkbroadband" is better (and indeed, I know the people and the network behind it and can vouch for their technical know-how) .... but that still doesn't make "thinkbroadband" or any other test immune from the points raised above because they can't have control over all four ("thinkbroadband" have 100% control over d & e, and attempt to influence c by technical means although that's a bit of a black art ....... "speedtest" merely link to tests hosted by third parties, so they have limited control over quality).