I was smelting ore not too long ago, and I noticed that - even though all ore was placed there before lighting the fire - ore that I placed first and left for a bit melted faster than ore that was placed right before lighting the fire.
Basically, I put my ore in the smelter, ran back down to get some more, and put that in before lighting it. The older ones melted with only 3 charcoal, while the newer ones took 4.