It seems like Quick Synthesis has a built-in 5%(?) fail rate, regardless of your gear. Maybe it is just a penalty for using quick synthesis in general.

But, I still feel like the fail rate is too high.
As a level 50 Culinarian, I should not be failing an synths in the 1-30~40 level range, especially with full HQ gear (don't have materia melds yet). If I spent the time to do it manually and would never fail, I should never fail at quick synth either.