What are all the factors that increase the chances of crafting double resources?

When crafting items, there is a chance you can craft two items instead of one.

For example, when smelting iron ore into iron ingots, there is a percentage chance indicated on the right side of the crafting interface that displays the chances of creating two ingots instead of one. In the case of iron ingots (and other low tier items you can craft), there aren't any other resources needed to craft it.

For higher level items that require an additional resource to craft, the chances of creating an additional item increases based on the additional resource used. An example of this is steel ingots. They require:

• Iron ingots
• Charcoal
• Flux

Where the flux used to create the steel ingots impacts the chances of creating an extra steel ingot.

So, obviously the type of flux used impacts the chances of creating an additional ingot, where using more rare/expensive flux increases the percentage. The same can be said for all other items you can craft that require extra resources. But what else impacts the percentage to create an additional item? Low tier items such as iron ingots don't require an additional resource to be crafted, yet they have a chance of producing an extra item, so what influences this?