Grip. The small size of the PX4 makes you focus harder on your grip. The more comfortable, larger P09 lets you get away with more slop in your grip and still "feel" good. Same reason a lot of guys shoot a G19 or G26 better than a G17. Dial in your grip on the P09 paying extra attention to your index points and pressure (consistency is key) and I'll bet those "flyers" disappear.