Because you just suck? I was wringing out my USPC9 this afternoon, bone stock with Mepro night sights, and rapid fire at 10 yards I group only nominally looser than my P229 with a Robert Burke action job and an RMR. Go figure. The biggest difference for me is placement of my support hand thumb. For me, there is not a good place to put it on my HK vs on my Sig. If I don't have careful placement of my support hand with my wrist intentionally broken downward, I tend to pull my shots down and left. I spent most of last year fine tuning my support hand placement. I also have to intentionally work on keeping the pad of my trigger finger on the trigger re the first joint. That also makes me pull shots. Each pistol grip is different so I have to work on hard to shoot consistently across platforms.