Well, one of the reasons that the page quality can go down, even when in the slab is because these slabs are not hermetically sealed. Air can still get in there and eventually cause the same kind of page deterioration and aging process that would normally occur. Maybe slower, but still there.
Plus, you never know when re-grading a comic if the same person grades it. They could see it slightly differently, especially if it's borderline.
Nice issue though. Used to have one years ago, but probably only in something like 7.0 condition.