There are couple good answers for this actually.

First off the storm troopers are not the same clones of Jango Fette that were used by the republic from the prequels. At first they started using other templates not just Jango's. I think there was something about the template eventually deteriorating or something. Then the Kaminoans essentially stopped complying after they saw the republic turn into the empire, they eventually replaced most of the Clone Troopers with recruited Storm Troopers. This would explain why they are no longer the super soldiers they once were.

But this still leaves one on answered question, why does Ben Kenobi remark that only storm troopers were that percise?

One thought was that he was not aware of the replacement as he had been hiding and his intel was probably pretty limited. This is probably true and it especially makes sense that the Empire wouldn't want this to be public knowledge. The reputation of the Troopers at this point was probably more effective than the troopers themselves. But you'd still assume at least the ones being utilized for more than just keeping the piece would be better trained than that.

Then there is a theory I completely hate, but totally something which seems to be kind of the way George was leaning at one point, is that force is seeking balance itself and is essentially making things work more favorably for those fighting to restore it. Of course this brings up the whole debate about what balance really means, but that's another argument.

My favorite logical theory is that Vader actually has ordered them to miss in at least one scenario. In a New Hope when they are trying to free Leia they were actually ordered by Vader to let them escape. Think about it, first off the tracking device they put on the Millennium falcon is actually how they find out about the rebel base on Yavin. Letting them escape got them more information out of Leia than the interrogation did. It also explains other issues like why Vader continues walking towards the slowly closing door when he could have easily run faster and gotten through. After this event they tend to be better shots, they miss largely because Luke becomes skilled enough with a lightsaber and the force to protect them.

There is also good reason to believe that Vader knows Leia and Luke are his children but does not want the Emperor to know this. He clearly comes around by the end of the Return of the Jedi, and knows he is Luke's son. He knew Organa so it is reasonable to presume between the force and some moderate detective skills he could figure out that Leia may also be his child. Perhaps this began changing his priorities. He might have come up with reasons to try to insure their safety after this.