well......one answer is that the speedtest.net test is rather unreliable
try this one
thinkbroadband :: UK Broadband Speed Test
but remember a specific download is only a part of your download throughput, and that you may be limited by the upload speed of the site you are downloading from.........and if you're a regular downloader your ISP may have throttled your connection
what sort of file are you pulling down, and who is your ISP