I don't believe this is a logical inconsistency, because furnace recipes like this mean reducing your fuel needs by a considerable amount to the point of absurdity. Instead of one coal and then some cooking 9 ingots, this let's you cook 8 blocks with one coal, and that's honestly really cheap.

I think taking a look at this video may be of some benefit as you start to expand your creative horizons.
youtu.be/y8GzZfgHZvE