Most of my fixed sights guns shoot POA for me except a model 10 which gets shot only on days when I feel incline to do lots of Kentucky windage.This being said,the most important factor is not the sights being fixed or adjustable but the relationship between front and rear sights.In other words,if the sights fit your eye,you're on.
The other factor that might happen is when you raise the coefficient of difficulty,you are more carefull with your drill of aiming,squeezing trigger,etc,which will result in more accuracy.Fixed sight are normally not picked up as easily as adjustable.You have to concentrate a little more while squeezing your shot.As an example,just try to post your target 5 or 10 yards farther than where you usually shoot and after a little while,your scores will probably be pretty close to your usual scores.But you'll probably shoot less rds since you'll tire out more quickly from concentration.
Qc