Two points I'd like to toss in:

1) There is a factor called "risk" that is referenced in quite a few advanced synthesis abilities, but is (afaik) not very well understood. It does not appear to be orb-color (Preserve references "beneficial synthesis conditions", not risk). It does not appear to be aetherial sparking (Grand Design references "reduces aetherial sparking", not risk). Some of the abilities that reference risk are Harmonize ("Reduces Risk") and Comfort Zone ("Temporarily Reduces Risk") as well as Flawlessness ("Temporarily Prevents Increases in Risk").

Unfortunately just what risk is about is not particularly transparent right now. However, if there is a persistent synthesis attribute of risk, it could go some way toward explaining apparently odd behaviors. If the risk is high, synthesis steps may consistently blow up, while if risk is low, things may sail smoothly.

This is just a hypothesis of mine at this point.

2) It seem pretty clear that quality is not the only factor that goes into determining HQ likelihood. Trying to map HQ chance (and degree) strictly to quality is a pretty inconsistent trend. Sure quality affects the outcome, but there really must be some other factors at play, which are not (or not generally) well understood.