I don't have an explanation. But i would add that this same scenario of bolts vs no bolts also went on in the 421, 422, 423 series of MI drivers.

A WAG(wild assed guess) of mine is that there may have been some sort of issue with the adhesives used to bond the ferrite to the plates. Or, perhaps a combination of adhesive/thermal issues that caused separation and eventual pole shift in the motors. The bolts would certainly ensure that the assembly stayed put thru a variety of conditions.