I think (and others will correct me if I am wrong) that BIAB methods are less efficient then a mash tun and sparge method but as I am not terribly familiar with BIAB it is only an impression from a variety of online reading.
A
Yep, you're wrong
Seriously, it depends how you do it. I've done a BIAB no sparge 'properly' twice and got higher efficiency than the recipe predicted.
As Clibit suggested the low FG is probably because a high proportion of the wort was fermentable.
No harm in being critical of your brew, its the way to improve. The end result will be beer though :-)