The velocity differences aren't a surprise.
When the factory labeling process is based on statical sampling,
management establishes acceptable defect rates,
defect levels, level of confidence to be applied to establish the sample size.
Profit is job one, expense kept to a minimum,
so 400 shots are all that are actually fired to label a batch of 28,000 cartridges.
That leaves room for less than stellar quality product to slip by.
Welcome to the assembly line lottery.
We pays our money, we takes our chances.