All powders are subject to lot-to-lot variations. From what I understand this is because of variations in atmospheric moisture.
With canister powders most manufacturers minimize the difference by blending different batches of powder, but it's virtually impossible to exactly match each lot with all previous lots. Usually the variations aren't much, though I've seen a difference of 3% between lots of some very popular powders.
Plus, sometimes the actual manufacturer changes. One powder distributor (not manufacturer) sold a certain mil-surp powder for years, but finally ran out. They contracted with another company to make the "same" powder. Well, the first lot wasn't too close. I bought some for use in the .223 and found it only took 26 grains to match the velocities of the 28-grain load I'd been using for years! That's close to an 8% difference. 28 grains caused leaky primers....
