As my feeble brain understands it, one of the major factors is velocity. Working up the most accurate load in a rifle basically means finding the velocity that results in the bullet leaving the barrel at the top of that swing. When you change bullet weights, the velocity changes, as does the relationship between the bullet and the lands. This means that we no longer have the bullet leaving at the top of the swing. Check out this article from the California Varmint Callers Association.
http://www.varmintcallers.com/accuracy.htm
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You know the rest. In the books you have read
How the British Regulars fired and fled,
How the farmers gave them ball for ball,
From behind each fence and farmyard wall,
Chasing the redcoats down the lane,
Then crossing the fields to emerge again
Under the trees at the turn of the road,
And only pausing to fire and load.
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