For example an Ancient Black Dragon has 15 (+2) Wisdom. The formula for passive perception is 10 + WISMOD + Perception_Proficiency_Bonus. For a CR21 the proficiency bonus is +7, so it should have a passive perception of 10 + 2 + 7 = 19. However, it's listed as 26.
This is 7 higher, so my suspicion is that some creatures add double proficiency, but I haven't verified that this is a hard and fast rule (is there such a thing? :p) or just a coincidence.
What I'm wondering is what to do if the Dragon has something cast on it that buffs it's wisdom enough to change the modifier. If it really is a double proficiency bonus for the Dragon it would make sense to modify its passive perception as its Wisdom changes. However if the extra 7 being equal to the proficiency bonus was a coincidence it might mean that listing a skill in a stat block totally decouples the skill value from the associated attribute.