The common sense you speak of is your reality and experiences. Others have more well-rounded view of reality.
For ex:
How would you feel if you were a 22.3 employee with 12 years PT, 6 years FT- and another FT employee of 10 years PT, 7 years full-time, and bumping you? When the job that the person with more FT/less PT originally obtained, was not posted for bid in your building (different UPS region, but same local union and annual bids mesh together) ?
That's the awful side-deal between union and UPS that changed my mind from your "common sense perspective" and maybe something you are neglecting to see.
The FT over PT deal is enforced across-the-board, so to speak, and not enforced selectively to your taste, basically.