Quantity counts in schedules include all instances of a item in the project. There is no way to filter it to exclude items on a certain layer system (such as existing).
If you want to exclude existing items from the counts, you will need to create a separate schedule entry for the existing items. You will need add an “E” or something similar to the name of the entry.
Uncheck the “Include in Schedule” toggle in the top-left corner of the schedule dialog. This will remove the item from the schedule when it is printed on the drawing.
This workaround works for diffusers, light fixtures, receptacles, low voltage devices, switches, junction boxes, and pipe symbols.