No you didnt miss anything it really does work that way. I never really find having sprites and such set to global a good thing. I usually right click the layout in the layout menu and clone it.
The only stuff I would mark global would be things like mouse and keyboard function object and any object that does not show in the layout. and if you do clone the layout make sure to delete any gobal objects so you dont have two instances running at the same time. so here it is in list form:
Dont make visible objects global.
Clone the layout instead of making a new one.
Delete any global Non-layout stuff from the layout(Mouse and keyboard,Function,etc)
The reason I never have visible object global is because if you go to another layout it will stay in the original layout and you will have two instances going at the same time so its better to keep them not global and just have an instance in every layout.