Rigidbody.sleepThreshold?

Question

C++ / C#

How does Rigidbody.sleepThreshold work? It is written that this is the limit of normalized energy, below which a solid body is considered to be at rest.
No sooner said than done. I create a cube of unit mass. I put zero resistance. I remove gravity.
I pass the threshold speed to the cube: everything is fine, the cube moves uniformly (by inertia).
If the speed is reduced, the cube for some reason starts to move. But the most interesting thing is that the friction force begins to act on the cube, moreover, at some other lower threshold.
What is this sorcery? And how to deal with it correctly?
ps
In all cases, IsSleeping == false, even when the difference with the threshold value is more than 10 orders of magnitude.