it took some expert restorer to be able to separate a stack so well. I'd have to think that it may have to do with how much or how little paste was used to adhere each successive poster to the stack. The more paste, the more the posters would be intertwined with glue. However it would seem that they must have used just enough paste to get the new poster on and because glue disintegrates into dust as time goes by, that there must have been "pockets" of the paper that had no real adhesion and once bathed released just simple glued areas easily..
must have been very expensive to have that work done...., but of course, worth every penny