I just got an idea that maybe you need 5 ingots only when you are highly unskilled. When you master the craft, you don't make silly mistakes anymore and don't waste ingots.
The thing is that while making a blade, you may break it in the process and so the ingot is wasted. Also, sorry if messed something up stock-ingot i don't know the crafting in PS well.
But if it is the case, i think it was made in a bit wrong way. It is player friendly that you know always how much ingots you will use with a given skill. But say that everytime you try to make a blade, you take one ingot for a dagger for example and try, you can be extremaly lucky and success at the first time, but you can as well break 10 ingots and nothing. That would be more realistic, as more direct touch with the rules of reality, but less player friendly, because less predictable. But I'm all for it.