The above posts are correct, but perhaps I can elaborate a bit.
With a GI profile, thin walled barrel, each shot generates harmonics in the barrel from the torque of the bullet entering the lands and grooves and starting to spin.
Keep in mind that constant pressure from the explosion that set the bullet off is forcing that round into the lands and grooves at enormous forward pressure and that is resulting in torque.
That torque is easier to cause subtle changes by vibration the further it gets from the base, which is where the thinner barrel is anchored to the action. Much like a fulcrum, the more distance you get away from the point of contact, the easier it is to make changes.
With a heavier barrel, you are adding to the stiffness of the barrel, which reduces the ability of the harmonics to make those subtle changes.
Take a drinking straw in two hands and twist it in your hands with one hand clockwise and one counter-clockwise. Now try to do the same thing with a section of garden hose the same length and you will get the picture.