This is a very basic explanation from me.
A "new" mainspring would be a completely straight and flat strip of metal
It is then given arbor hole/bridle etc and "curved" to form a loose coil we know of.
As that coil is tightened and loosened (winding/unwinding) over time, the spring loses its elastic properties and becomes more of a condensed coil, and this results in a shortened "power reserve" as the coil does not unwind to the full extent that it did when new and is not pushing against the barrel wall as hard as a new spring would.
So, when an auto winder starts winding an old spring, there is very little resistance and while the auto works turn the spring, it's not putting in much power.
The auto winder continues to wind until the bridle slips against the barrel wall, and that is now at full wind, somewhat less energy stored compared to a new spring.
A new spring will need a lot of turns to compress the coil until the bridle slips.
A tired spring will not need as many turns before it slips. Therefore the loss occurs due to loss of elasticity in the spring. Not work hardening.
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