OK. In BeerSmith 3, the ability of the yeast to turn those sugars into alcohol is capped by the individual yeast strain and the alcohol tolerance of that yeast. US-05 has a maximum alcohol tolerance of 10% in the BeerSmith profile and an attenuation of around 76.5% (average of max and min) which when applied to a gravity of 1.106 would give you a projected FG of around 1.025 with an alcohol content of around 11%. Since this is above the 10% max for the yeast, the program caps the ABV at 10% and projects an FG of 1.035 (actual value varies depending upon mash conditions).
The amount of yeast you pitched does not matter to the software, which does not know this information. What does matter is the specifications of the yeast strains which imposes a limit on the amount of alcohol before the yeast will not function (right or wrong is another issue all together).
In your recipe, edit the yeast by double clicking on it in the design box. Change the maximum alcohol tolerance to 12% and see where the estimation of FG comes in. If your wort was all grains of extract, I would estimate that it would be somewhere in the range of the 1.025 I calculated above. (again, mash conditions will affect the actual number, which is why the actual recipe makes a difference). If you have some simple sugars in the recipe, then the number will be lower than that.
Edit: Your actual value of 1.020 is not out of the question, since the maximum attenuation of US-05 is 80% and going from 1.106 down to 1.020 is around 79.7% attenuation. Well within the capabilities of the yeast. Thus further indicating that the software limit of 10% for that strain may be incorrect and providing an erroneous FG.