Can't really argue with you on that without any knowledge of the drawings and parts lists for the different part numbers. If you have any additonal details on the difference between a 12645045 starter and a 12655055 starter (other than origin of manufacture), we'd all be more educated on the issue if you shared with us.
I will say as someone that works for an OEM, there is a considerable amount of cost associated with changing a part number(drawing, parts list, first artical test, environment qual, etc......) not trivial. I would think that they would just change the revision, group number, or production build sheet if less grease was the only difference.