The Select All Instances tool does in fact select all similar elements in your ENTIRE model. If you are modifing model elements (stretchers, lavatories, windows, walls, etc), or annotation elements (dimensions, keynotes, tag, etc) and decide to ‘select all instances’, you will be modifying elements in the entire file, all floors and all sheet views.
Sure it'd be nice to Select All Instances in this view only, but that's not the case... yet.
In AutoCAD terms, Select All Instances is like CAD’s Quick Select tool across the board. Quiet useful... and potentially harmful.