@MattL, it’s baffling until you look at the underlying technology. You have to think of all the on and off transitions in your house as a bunch of points in a many-dimensional space. Some of the dimensions include per-leg power, phase, maybe some kind of measurement of switching speed. Sense detects devices by dividing up the points into well-defined groupings. Every once in a while two of these groupings might grow too close together and Sense combines all the points. Or they try a “better” dividing algorithm that combines the two.
More informed speculation on how Sense native detection works here: