Undersized bearing journals result over time from natural wear or improperly lubed hub encasing.  The slop in the bearings result in unbalanced duals damaging the bearing and the hub and produces excessive wear on the tires.  Left unchecked, undersized bearing journals ultimately cause the bearing to overheat and finally damage the spindle.

 

This picture depicts what happened after a bearing failure caused the axle to flare the threads and damage the journal.