I see the following condition a lot:
1. Speed test provided by isp (internal to its domain) results in speeds at or above subscription rate.
2. External speed tests, such as toast.net or cnet.com, results in speeds markedly lower, sometimes by 50% lower or even more.
Moral: once you venture past the gates or perimeter of your isp, all bets are off.
What if your isp does not offer its own internal speed test utility or service? You can still measure download/upload speeds via ftp between your PC and the online storage space the isp provides for your account (many do).