Wow I didn't even think about the recycled yeast aspect... I know I've harvested yeast a couple of times and reused it for one or two generations, but as a rule I can't be bothered. When I do, I just pick a load of slurry from the bottom.
I insist that those brews were pretty good, so if there's a parasite they're welcome back in my wort
(jk please stay away contaminants, *knock on wood*). But I'll still try to pay attention to that.
I sometimes used the unsealed technique, not locking down the fermenting bucket during high krausen, which is said to yield lower FG, but not by that much. Of course one could see an infection risk there...
Anyway, I guess y'all are right and there must have been some error in my measurements, even if attenuation was a little higher than advertised. Looking forward to my next brews to confirm