As the naming suggests "global variable" is global. So you cannot treat it separately for various layouts - it is global for entire projejct.
If you want to do a kind of global variable for the particular layout, you may do a "global" groupd in event sheet assigned to the layout and set static variable for this group. It will be visible for all subgroups as well but not for other layouts.
Another thing is that for high scores you may want to use LocalStorage.