There is a good chance that one load is generating more pressure and/or velocity than the other. Recoil is generated by 2 things in the same gun; the weight and velocity of the shot+wad exiting the barrel and the weight and velocity of the gas leaving the barrel. Same situation as a rifle. It is completely normal for two wads to generate different different velocities and pressures with the same load. It is also possible, with the same velocity, to generate different pressures. With modern powders the change in pressure affects burning rate. The result is that the shot+wad can be travelling at a higher velocity and therefore generates a higher contribution to recoil. Similarly, a higher pressure generated by on wad vs the other, even with the same powder charge, means that the the gas is going to have a higher exit velocity which yields a higher recoil contribution.
The assumption isn't at all true that simply switching wads in a load will yield the same results. In fact, if you look through loading manuals and the pressure data load reports often posted here you will see there is normally an effect (sometimes significant) on pressure and velocity when swithing wads.
It should be kept in mind too that the shot velocity and gas velocity are quite different, even though the one is not measured. Since the kinectic energy varies increases with the square of the velocity, the contribution of the gas veocity increase is relatively larger than the shot due to the magnitude of the energy increase due to the velocity increase.
That's the reason that low recoil loads based on single base powders work. Small wt. loads due to the rapid combustion of the single base powders generate their pressur quickly and then the shot is accellerated down the barrel. By the time it gets to the muzzle, the combustion is over and cooled due to expansion so that the shot and gas exit at much closer to the same speed so that the gas generates less of a recoil contribution. It is very similar to the pressure curve and performance of black powder.
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