I think it is because of the Q-factor. Shimano 12-speed chainrings have a different Q-factor (+2 mm) compared to Shimano 11-speed. Because of this, you need to add a washer. Pass Quest makes their chainrings universal for 11/12-speed, but the Q-factor matches the 11-speed Shimano chainrings. I added a 1 mm washer because the FD-8150 front derailleur does not have enough adjustment range.