There are too many variables involved to predict a FG unless you have repeatedly brewed the same recipe over and over at home and know the range you will get. As stated above, things like mash temp accuracy, grain to water volume, malt used, conversion efficiency, wort aeration levels, yeast vitality/pitch rate/health, post boil volume accuracy, and fermentation profile are just a few that come to mind.
As Denny said, don't take too much heed in hitting an exact FG. In a perfect world we would hit it every time but Lord knows that doesn't always happen. I consider my brew days a complete success if I am within 2 points + or - of my OG and FG expected.
After all we are just homebrewers....