Not knowing anything about this but I'd sayit would be to stop any play in the gearset, thin shims would be placed equally at each ends of the shaft to stop any float in the g box... Correct me if I'm wrong, I don't find it hard to believe that people are doing this not every motor is identical but close enough to get away with leaving some slack they are mass produced and time is money so they would not go over everything with a fine tooth comb
P.s dhz where's my review on the psto