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If you test a number of different manufacturing lots of the same powder you'll find that much variation, or close to it, in many. In general, manufacturers of handloading powders try to keep lots within 2% of the "target" burn-rate, but it doesn't always work out quite that small--especially since relative burn-rate (as I hinted at above) varies with the cartridge and bullet.

Also, MOST new batched of the "same" powder vary some, due to variations in manufacturing conditions. From what I've been told, this is primarily due to differences in atmospheric moisture during manufacturing, but can also be due to the variations in the organic chemicals used to make the powder.

The common solution for this is to keep some of the previous batches of that powder, which turned out to be faster or slower-burning than the target burn-rate, and blend them with the new batch to "adjust" it's burn-rate. I've done this myself when two batches of canister powder vary noticeably in burn-rate.

Ammo manufacturers don't usually use blended powders. Instead they buy unblended powders in bulk, and the ammo company adjusts the powder charge to match the pressure/velocity of previous batches of ammo.