"Can someone explain why you would arbitrarily change the OAL in a revolver?"

What does the actual dimension in inches have to do with anything anyway? What you want is the bullet to sit in the correct location in the case relative to the crimp groove or cannelure. The actual number usually means zero. If your using proper load development procedure, the final OAL means precisely zero as long as the finished round is functional. The same goes for rifle ammo. You almost invariably want the finished round to be as long as possible and still cycle in your gun and not have the seated bullet engage the rifling of the barrel. This can sometimes result in a cartridge which is longer than the listed maximum OAL but who cares as long as the finished product functions reliably in your gun. The dimensions listed in loading manuals were developed in order to standardize ammunition so that commercial loads will chamber & fire in any gun built to standard dimensions. When you are loading, your building ammo for YOUR GUN and not anybody else's.

Bruce