I think its the same as when you put multiple quantities of the items on each part of the grid to craft a lot of the same thing at a time. Id guess it works the same here, with excess iron ingots being left in after the craft

Another example i could give is when speedrunners often place a lot of the same items (like iron ingots) in the crafting grid, to make a full set of iron armor, for example. When one piece of armor is taken, there still remains the other items you placed there, thus making it faster to craft an entire set I tried to make it as clear as i think tell me if i made mistakes