What says the magic isn't making it more efficient? We are talking about scientific magic here. Not to mention saying it is more efficient is simply a way of explaining why it creates something from nothing.
If it truly did create something from nothing, we'd be able to repeat the experiment. It's just a processing method, since we can't redo this without constantly mining out more ore.
Another way to think of it might be that the Ore acts as the catalyst necessary to induce the production of dust. Okay, not the catalyst, since it gets used up in the reaction. The point is, since we can't macerate Dust into 2 Dust, we need to constantly supply it with more Ore, so it's not just energy we're processing.
Unless, of course, we're using UUM to produce our Ore. But UUM takes lots of energy to produce.