Why does load data listed in various manufacturers' loading manuals differ when loading the same cartridge?

The size of the specific firearm chamber, throat dimensions, seating depth, bullet profile, propellant variances and crimping depth can all contribute to variations in load data. The data found in the Hornady loading manual represents actual results derived in the Hornady Ballistics Laboratory. Hornady recommends that reloaders always start with the “starting load” and work up cautiously until they achieve the performance they're looking for. If pressure signs become evident, stop immediately and reduce the load or try another propellant.