How Does The Level Of Crafted Items Work?

When you craft an item, it has a range of 3 or 4 possible level minimums (for example, level 15-17). How is the minimum level determined within this range? Is it equally spread out, or are some more likely than others?

 

It can make an item within this range. your ingredients can help increase or decrease the level, at least to my knowledge. It appears to be normally spread out however.