I have recently switched ISPs and I have been keeping a closer eye on my broadband speeds than usual. I have been surprised to see speeds of less than 2.0Mbps and sometimes less than 0.5Mbps where I was expecting around 7.0 Mbps.
Today I had a moment when most of the other attached devices are off and I thought I could get a clean test. I was shocked to get a 0.4Mbps reading. I switched to the alternate tester for a comparison and 2 runs both gave readings around 6.75 Mbps. Another try with the new tester again gave a much lower speed of about 1.5 Mbps.
It seems to me that the differences are very much more down to the tester than to any real differences in the network and that the alternate tester is much more accurate.