A lack of practice is definately a factor and then there's that massive shot of adrenaline when you are getting shot at. Unfortunately there is no real world practice that simulates being shot at. Sure you can do things to increase your adrenaline, you can have paintballs, airsoft, etc sent your way. You can be under the stress of competition and the have the surprise of a timer going off in your ear, but I'm sure that nothing prepares you for defending your life and those around you with real bullets coming at you. You can be a great shot at the range, hell you might be good at shooting on the move but you are in a safe enviroment. Another factor is the amount of ambient light in a shooting encounter, they usually occur in low light condidtions and not many people practice that type of shooting. If you get the chance do it.