I'm starting to think it's all luck & level.
Craft an item wearing my best gear: I get +300 quality.
Craft same item with no gear: I get +300 quality.
Craft item 10 levels higher than me: I get +250 quality.
As I approach the level, the quality improves.

(Note, I'm using the same skills/sequence each time). If there is a difference, it may not be so big to go: WOW, I need to have that!