Another thing to remember is most of us shooting in competition are shooting are shooting freshly-cast ammo with fresh lube on them.
Period US expanding ball ammunition was put up in paper cartridges in paper bundles of 10 and put up in wooden crates and shipped by train, boat, and wagon. Thus they were exposed to a lot of jostling, extreme temperatures, and humidity. The British discovered that tallow caused oxidation of the lead which caused the bullets to grow slightly in size, which is one reason why they switched eventually to 100% beeswax as their lubricant.
US bullets were put into paper cartridges, but the bullet was removed from the paper prior to firing. It was loaded "naked". If the cartridges had been exposed to high temperatures, the lubricant would melt and be soaked up by the paper cartridge. Thus the lube was lost from the bullet.
This means that you could be shooting ammunition that was possibly corroded, and possibly with little or no lubricant still on the bullet. In addition you had variation in bullet sizes due to wear of sizing dies and different dies from different arsenals and commercial sources. In addition you had variation in bore sizes.
Steve