At first glance the latency 'dip' appears as network congestion during the test, some type of QOS, or if this is happening on all tests, could be power fluctuations within the first line node. Or any number of other issues actually.
As far as speedtest, it's not showing the latency because it discards such findings. Why is anyone's best guess. But great for ISP's since it will not show issues such as you have pointed out in testmy.net results.