Its something odd with the connection, if the package purchased has a 75 Mbps guarantee with burst above then it all depends on how the bursting is detected by the ISP.
Our test and speediest.net both appear to be being impacted, e.g. one of your speediest.net graphs shows high start and then a drop away. The difference in the end figures is easily explained.
Our average is a median figure for the test.
Speedtest.net rejects 20% slowest, and 10% fastest and then picks an average (not sure if its mean or median), this often gives slightly higher figures when a test does not get a flat line.