1) variation in alloy content
2) variation in melt temperature
3) variation in mold temperature
4) Different mold cavities
Until you are shooting well over 100 yards, the weight variation is of no consequence.
As far as your gun is concerned, lead bullets of, say, 120-125gn are all the same. Difference in friction and hardness of copper plated and jacketed bullets has a greater effect on pressure than +/- 3% weight variation.
Do remember, while thinking these thoughts, that the history of shooting (500 years?) is cast lead, so most of these concerns have been worked out already.
If you can find them, you can get swaged lead bullets that are 2/6/92 alloy and VERY consistent in diameter and weight.
The biggest problems with commercial cast lead bullets are:
1) folks not slugging their barrel so they know the minimum bullet diameter that will work
2) an alloy so hard (18-22BHN) that the bullet can not obdurate (be squeezed up) to fill the bore, making an oversize lead bullet even more important. Most handguns (even .44 Mag) can be handled quite well with 10-14 BHN alloy.