Some of the spray will be forced over the top of the car when the nozzles are properly aimed depending on the speed of the car. The wind force up and over the windshield is significant at speed.
I've performed this experiment - aimed them with a ruler to exactly where they are supposed to be according to the FSM. At highway speeds, even though most of it is in fact hitting the glass where it is supposed to, some spray will also hit the windshield of the tailgating bozo behind. Kinda funny when they get pissed off, race around & hit their washer, giving me a free wash.