I have been brewing for almost 2 years so still consider myself a noob and learning. I BIAB, use Brewsmith software and TILT's to measure gravity. As I BIAB I have a brewhouse efficiency of about 70% so manipulate the amount of grain used to give me the correct OG. However I quite often find that on brewday while the OG is within a point of where its supposed to be, my FG quite often ends up 5 points lower than expected. What might the reason be?