I notice that Speedof.me consistently reports much lower results than Speedtest. In my case, download speeds are always around 30 mbps or better with Speedtest, much slower with Speedof.me. I have run tests with both, one after the other, using the same computer and connection. 7 mbps is the best I have had with Speedof.me, often it is as low as 3 or less. 

I realise that different servers will produce different results, but should they consistently be this different? What is the significance of this, if any? Which should I believe?