I believe ISP's generally only test and define usability within their own network. So you'll get routed directly along side of the pipe your connected to. Entirely useless for testing real world usability.
Flash IMO is a total waste of time in the current configuration means of testing. I ask myself, when do I use flash to browse or download, not very often , at all. Uploaders sometimes use flash for multiple files, there are legit reasons, none the less not for testing throughput.
Run a few ping tests to google.com and testmy.net , also you could use a trace route to find the weak link. If you choose post the results here excluding your public IP.
50% of paid bandwidth is unacceptable lets find the issue.