Originally Posted by Mr Downtown
^Could the difference be where the tower crane is mounted?
I think it just has to do with the way the floors tie into the core. Certain buildings probably can't be properly tied into the core if it rises above the surrounding floors. This would explain why the cores on steel towers always take off since they have steel plates embedded in them which obviously can be welded onto regardless of how many floors of concrete are above them.
Think about it, if you have a design that requires rebar or post tensioning to tie into the core, you don't want to run your core up 10 floors and have bundles of rebar and post tensioning cables just hanging there waiting to get bent or snag something and cause an accident.