Hello Mark! Ammo manufacturers are able to reduce how much recoil the shooter actually perceives by manipulating their shotgun powder formulas. (Felt recoil and actual recoil energy are very different things – in effect, how suddenly all that recoil energy is distributed all at once may be drawn out.) Manufactuers can achieve this without actually reducing muzzle velocity, although low recoil shotshells are known for their frequent inability to correctly cycle a semi-auto.
To give you an analogy, suppose a boxer were to punch you at his usual speed. Now, imagine he punches you again but slower, exerting the same amount of energy but not as suddenly. The slow punch is more analogous to a low recoil shell – same energy, but distributed in such a way that it's less jarring to the shooter. Of course "felt recoil" is necessarily a subjective thing, impossible to quantify. Only the shooter is qualified to note the difference!
I hope this helps!