The depth of the groove is beyond the means of most of us to alter.
This is because most of us don't have a selection of router bits at hand and only shell out the money for a single tri-wing slot cutter, arbor, and bearing. The depth of the slot can be varied by using different sized bearings on your router arbor, but most of us only want to buy 1 bearing as necessary.
The bearing that came with my slot cutting arbor creates a 1/2 inch depth slot... which is more than is necessary.