Many things can cause the ABV or estimated SGs to be off. How do you know the ABV is off?
The things I have encountered with the BS SGs and ABV estimates vs the BS calculated final ABV being off are many and varied.
I have confidence in the BS software. I always say" There is a reason for everything". The fun begins in the resolution of the mystery.
Welcome to Home Brewing.
1.) Equipment Profile must accurately portray your brewing equipment and varying brewing processes. So a couple of different equipment profiles are good, although to keep life simple I use just a few equipment profiles knowing that on any particular brew I can tweak it via the individual BS recipe file.
2.) Boil Rate in some areas vary more than others, also the weather/time of year effecting the density of the wort. And it is difficult to nail.
3.) Water volumes are crucial. Must may attention to what BS expects throughout the brewing processes. I have had issues with the BIAB squeeze. It also can vary. (Note to self: don't want to squeeze if using darker grains). Grains and hops water absorption and consumption amounts can vary.
4.) Grains yield established calculated SG values. Normally they be off just a little, but it could be your cuprite.
If I was to guess, I suspect water so where in your process has changed. For me the adding of more wort to the boil because I could squeeze it out evolved into my similar issue of the past. While more wort is good if you have it available, it will thin the final SG of the boil, unless you extend the boil. So watching SG values throughout the Brewing process gives you the ability to tweak during the brewing process.
It is all fun. Also after a decade or so of home brewing my taste buds have changed, and the lighter brews are now the targets instead of the accidents.
Good Luck!
