If I didn't know better (and I probably don't) I'd almost make a guess that when condition is Excellent, that some under-the-covers modifier gets baked into the equation.
I would venture to say that I have a <=50% success rate on Synths when Excellent. I see "Excellent" and say "oh boy.." cross my fingers...

Just my observation, YMMV. Just seems that way to me.