You account for 2 shots being fired from the sniper's nest. So how do we end up with 3 hulls on the floor of the sniper's nest?
Because three shots were fired from the Sniper's Nest. The first, missing-everything, shot (which didn't even hit the traffic signal's mast arm) was fired half-a-second before Zapruder resumed filming at Z-133, the second shot was fired around Z-222, and the third shot was fired at Z-313.