Why does velocity differ from what’s listed on the ammo box?

In order to comply with industry standards, Hornady publishes data for ammunition derived from testing with Sporting Arms and Ammunition Manufacturers Institute (SAAMI) specification test barrels. SAAMI specifies minimum and maximum chamber sizes and certain barrel lengths for certain cartridges. For instance, most rifle test barrels are 24 inches long. Due to variables associated with individual firearm chambers and/or barrels, velocities may be faster or slower than those published by Hornady.