It's not so much the volume difference. The shape of the front of the cab affects how the cab disperses, which causes a difference in what frequencies become standing waves, which in turn cancel different frequencies in the output waves. Basically it's like starting with one sound, and then cutting and boosting different frequencies. That's basically why cabs sound different even with the same speakers across the board. Plus straight front cabs are more directional, which is great for close mic recording because it limits the amount of sideways interference you have to account for (though not completely).