Villagers always put the maximum amount of fuel into a fire, which is usually way too much for the task at hand.
For example, one can smelt about 2 iron with 1 peat in the Furnace. If my Top-Up order wants 1 more iron, the villagers will put 4 peat (the maximum) in the Furnace, even though a single one would suffice.
They also tend to turn on the fire before the ingredients are in there, which also leads to waste, especially if another job takes their priority before they go add the ingredient.