The reasons are many. Just the answer you were looking for, right?

A speed test is not definitive, but more of a gauge of speed. When your browser communicates with another site it is not a direct connection. The communication passes through many other computers (servers) along the way. Each of these is called a 'hop'. Each hop increases the latency and generally decreases speed. Generally, the farther away you are from the physical location of the test server the more 'hops' there will be.

Another factor is the speed of the server and connection at the other end (speakeasy, Daves Computer Tips, etc). These are a few of the reasons that different tests give different results.

While Speakeasy may give you a close approximation of your connections speed, the best way to use these is tool to monitor your connection health. If you use a variety of services to test your connection, and remember those results, it can give you notice when performance isn't what it should be.

Hope that makes sense!