In a perfect world, every single round of ammunition would fire in the exact same way, every single time. However, powder and/or primers used, even the humidity or the temperature at which the ammunition was loaded can alter where a bullet strikes the target.

For that reason, ammunition manufacturers have an allowable variable, plus or minus, that their bullets will fire, also called standard deviation.

In reloading, one can use their most reliable load as the control by which the accuracy and consistency of all other loads are measured. Most serious reloaders have a good understanding of what standard deviation is allowed with their loads.