Coal acts like sticks in the furnace

I was doing a littke bit of mining to expand the basement of my home and i got quite a few resources. I used the furnace that i had with the coal in it to smelt some of the iron, but i was shocjed by the speed at which coal was being combusted. Every 2 coal pieces consumed, 1 ingot was created. (shouldn't it be 8 ingots per piece?) i dont think charcoal is affected. I recently realized this problem. Please, see the attached images.