Firstly, I'm not sure if this has already been posted, if so forgive me.
Secondly, I'm not sure how many people know this.
Yesterday while on doing Goldsmith quests I realised the amount of EXP I was getting per successful craft was different. It turns out that the more you use 'Bold' Synthesis, the more EXP you get.
You basically get EXP for every time you perform the craft. Try it for yourself, try to successfully craft an item using 'Rapid' Synthesis and then again with as many 'Bold' synthesis as you can and check the EXP.