a common misconception. climbs are rated in the YDS by overall difficulty, not by the hardest move. in the case of boulder starts, it is much easier to climb a boulder problem off the deck, with as many tries as you want and no fall consequence, than it is at the top of a climb (particularly if you don't haul your rack up until you are past the bouldery opening moves) so the difficulty of boulder starts is not reflected in the overall number grade for the route.