The higher the power the "shakier" it will be. Chances are, offhand, you just missed. Thankfully you didn't wound it. I keep my scopes set low until I need them higher. A 3-9X I'll typically keep on 4X until I need it higher. Do you practice offhand shots, or non resting shots? I encourage people to practice the shots they will take. Lots of people only shoot their rifle off the bench to make sure it's sighted in then wonder why they miss a fairly easy shot offhand. I practice all sorts of things. I'm even comfortable switching hands and shooting left on several of my deer rifles. Practice, more so than scope power, is key.
-Jake