More internal reflections reduce sharpness, a single thicker emulsion will have a higher number of internal reflections. If you use two different layers of emulsion, you can tailor them to contribute different qualities to the final emulsion, much like modern films. It's more complicated than just the gelatin being the same, there is the Ag content, Iodide concentration, etc. Gelatin is not a solid, if it can absorb chemicals (think developer, fixer) then the chemicals in it can move around as well. Since the older films have varied sizes of grains, chemical action will try to find an equilibrium of distribution (like a drop of ink in an aquarium of water). Newer films have grains sizes that are much more even, hence they are already close to equilibrium, and will change less over time.
Now, did I get that right?