Using recipebook in furnace with respect to charcoal creates different wood types

I had 6 oak and 6 acacia logs in my inventory. When I placed in one oak log and clicked the charcoal recipe in the recipebook. This moved the other 5 oak logs into the smelting slot, but also 5 of the 6 acacia logs. Then I had 1 acacia and 11 oak logs. I can move between different ratios of oak and acacia.