Can't sort by PartNo and retain Item No.

This is a new one to me. Could use some help please.

I go to sort my PartNo column ascending, have the 'do not change item number' box checked but it still changes/retains the item numbers.

I've tried unchecking, re-loading the table, re-loading the format, re-starting SW....   I don't get it.

TIA

SolidworksBill Of Materials