I've noticed the same phenomena. I think that that weight of the scope above the center line is part of the reason. No matter how much spring torque is removed with a tune, there is still some left no matter how imerceptable, and the addition of weight outside the centerline would most likely amlify that torque. It would also amplify any torque applied to the rifle by trigger pull, pressure from a good cheek weld, a shoulder muscle flexing in anticipation of the shot etc.
There's also the matter of of the mass of the scope dampening the recoil. This is true that more mass would dampen the recoil effect. But when you add that much mass to the rifleyou also slow down the movementl of the entire rifle giving it more of a window to be effected by body movment or some other outside force. When unscoped, or scoped with lighter glass the recoil will be a bit sharper and the movment of the rifle overall is faster giving less oppertunity for outside influence.
That much weight added to the rifle would also change it's center of balance and along with the overall slower movment of the rifle during recoil, may exaggerate any vertical stringing due to the rifle wanting to pivot to the heavier end.
Could be taken as the ravings of an idiot who doesn't have a clue what he's talking about... But that's my take on it! LoL

Then again, Ronbeaux has some pretty solidly valid points too!