BIAB with a fine crush?
I BIAB with a fine crush and typically get lower FG than beer smith predicts... is that why?
I would not pay much attention to any program that is predicting a potential FG. You can look at it as a potential finishing range, but don't take much heed in it. There are just too many variables involved that can affect a recipes final gravity (i.e. aeration levels, wort nutrients, grist, yeast health/vitality and pitch rate, fermentation temperature, etc).
The only way to really guess at the final gravity is simply to brew the same recipe a bunch of times using the exact same process each time and observing your final gravity ranges from those. Then you can predict for the future where that FG might finish at.