Because the QS interrupts the fuel delivery to unload the transmission to make the shift. The PC alone only controls the lower 4 injectors. You need another unit that controls the upper injectors to shut off all the fuel to make the shift.
or you can go with a Bazzaz qs4 stand alone unit, which uses a better qs switch, and cuts the ignition v.s fuel. and it is programmable for all gears, when you want different cuts for different gears. the higher the gear the less cut you need to make the shift.