The details are still debated by tornado scientists. There are a lot of processes around a thunderstorm that can rob the area around a tornado of either instability or vorticity. Many tornadoes have been observed to go away soon after being hit by outflow. The irony is that some kinds of thunderstorm outflow may help to cause tornadoes, while other forms of outflow may kill tornadoes. There were multiple tornadoes with only short separation, but the survey erroneously classified them as one tornado.
Definitions are arbitrary, for scientific research.
No tornado is necessarily insignificant. Any tornado can kill or cause damage, and some tornadoes rated less than F2 probably could do F2 or greater damage if they hit a well-built house during peak intensity. Both are major changes in sea surface temperature in the tropical Pacific which occur over the span of months that adjust large-scale weather patterns. When a tornado doesn't contain too much dust and debris, these subvortices can be spectacularly visible. Vortices may form and die within a few seconds, sometimes appearing to train through the same part of the tornado one after another.