My only guess is that instead of Blizz scaling up individual items they simply scale up EVERYTHING on you. Then when there's specific items that would be 'TOO powerful if scaled up' they simply have it set to scale those back down.
So instead of each item being given a value (ex: 1 for scale up, 0 for scale down), it goes:
---
If Value=0 (OP item if scaled) then ScaleDown
else DoNothing
end if
AllScaleUp
---
That way every item is being scaled up by the same amount, with OP items being scaled down beforehand to make up for the upscale.
Basically, it's Blizz saying "Your items have been scaled up, but these certain items have been scaled down to counteract the overall scale up we have given you and your items".
Then again, I have no clue what I'm talking about you probably should just ignore this post - Hopefully it helps though.