Well maybe a little bit of light on how networks work might explain it. You're connecting and testing your speed to a server ~50 miles away, which means fewer hops and the fewer number of networks that the packets have to traverse to get to the server. As soon as you connect to something further away you're hit with the bottleneck and latencies of all the networks you have to hit to download something from any particular server. So I can do a speed test and get 24-30MBit, but in reality as soon as I leave Comcast's network, I get closer to 18MBit.
A better way to determine what your bandwidth is to choose a to a particular server would be to do a speed test closest to the server that you're actually connecting to, which could be several hundred miles away from you.
Example: Comcast's SpeedTest in Boston
[url]http://www.speedtest.net/result/2797988639.png[/URL]
Versus Comcast in Chicago
[url]http://www.speedtest.net/result/2797990365.png[/URL]
Versus Comcast in Miami.
[url]http://www.speedtest.net/result/2797992814.png[/URL]
Even within Comcast's own network, you can see how it slows down the further away you go.