For two cider seasons now, I've used Safale S-04 on home pressed apple juice with a 1.054 OG, I then added some sugar, and they finished at 0.994 FG. I've used 3 other yeasts and gotten similar results, with 0.992-0.996 FGs.
I'm now trying to understand attenuation, and Safale S-04 has an (apparent?) attenuation of 75%. Ignoring the sugar, 75% attenuation on 1.054 should yield 1.013 FG. My actual results are over 100%.
What's the disconnect? How do I hit 100%+ on a yeast rated for 75%? What is the 75% number really trying to tell me? Is it a minimum or average? It certainly isn't a max.
To flip the conversation a bit. I just started in on 35lbs+ total batch of raspberries, plums and blackberries, and I was pondering what final alcohol level to hit, and thus how much sugar I'd have to add. As I started to read about attenuation, I started to wonder if I needed to over-dose on the sugar, in anticipation of only getting 75% attenuation and hence only ~75% of the alcohol I was shooting for. For now I don't plan to over-dose.