One thing to remember is, a global layer is not the same thing as a global variable or object.
Global layers are used to override layers on many layouts so you don't have to recreate something like a HUD on every single layout it is needed. This usage of global doesn't have anything to do with persisting throughout the game. Items in a global layout will be re-instantiated on start of layout.
Global variables/objects are used to give access to the same variable/object throughout the game runtime. These objects exist in all layouts as a single object so any change made to them is persisted when changing layouts. They are instantiated when the game is run and are not removed until the game ends. A non-global variable/object on the other hand is re-instantiated on start of layout for every layout they are place on and removed from memory (causing them to cease to exist) when the layout ends. If they still exist when the layout ends that is. According to the game engine, these are separate objects.