Could be that your connection is causing the test to fail.
https://testmy.net/stats/?&t=u&d=06092016&x=90&l=50&q=Jtemple
If you come back to your computer and the test has failed to progress it's usually an indication that your connection has dropped out. During the test your connection died, causing it to fail.
The next version of the auto speed test will recover itself when this happens and log the event. If your seeing this often then it's probably a good idea to follow up and see why it's happening. To avoid it from affecting the tests as much you can lengthen the test time to over 1 hour. Makes it less likely to occur. If you're currently testing with a 5 minute interval and you're seeing that it's happening regularly, you can almost certainly ascertain that your connection is having frequent disconnects.