As far as I can tell it's a purely vanilla bug that's never been fixed.
When I use a Power Armor Stand (where you go to repair/modify/upgrade Power Armor), it adds the combined weight of the armor plates to my inventory for the time that I am using it. It will show an incorrect weight value in the inventory after exiting the Stand, the value will then fix it'self if any weight changes are made.
It seems to be there in both Normal and Survival difficulty, with more severe consequences in Survival.