OPUPS Aren't meant to be perks
UG's undertaken by KL for the purpose of not denying boarding, are not meant to be perks and it makes a great deal of sense that there's no particular order to them as long as the system used is basically fair.
Unlike USA domestic carriers which hand out UG's, European carriers are more cagey about it so as to protect their premium brand.
Whether KL chooses to OPUP a high fare bucket Y pax or a high status flying on a lower fare bucket or a couple on their honeymoon, so long as the decisions aren't based on improper considerations, it's a better business practice than creating yet another opportunity for people to game the system.