An excerpt from The Final Truth:
“[Oswald] would carefully analyze why his single shot at Walker, which seemed a sure hit at less than a hundred feet, missed. The simple explanation was that the bullet Oswald fired hit the window frame, thus sparing General Walker’s life. But that reverses cause and effect. The striking of the window frame was the effect of the miss, not the cause. It was ‘how’ the shot missed, not ‘why’ it missed. With all the time he needed, having a fence as a stable gun platform,35 and using a 4-power scope from that close a distance, Oswald easily would have seen the window frame silhouetted by the interior room light and taken aim to avoid hitting it. He even told Marina that he took very careful aim.36 Yet the bullet went astray. Through his extensive knowledge of firearms and marksmanship, he deduced the true cause of that missed shot. He would not let that mistake happen again.”
Lee knew he missed Walker because he hadn’t zeroed (sighted-in) his carbine. Having learned his lesson, he used the first shot in Dealey Plaza to zero his Carcano. That’s why it ‘missed,’ it wasn’t aimed at the limo in the first place.