I don't try to fathom the logic behind nomenclature of early calibers and cartridges. For example, the .52 percussion Sharps is closer to a .54 and the Spencer cartridges designated .56/50 and .56/52 are interchangeable.
Might as well ask why we have .220 Swift, .221 Fireball, .222 Remington, .223 Remington, .224 Weatherby, and .225 Winchester when all of their bullets are .224 diameter.