There are several different equations for estimating ABV based on gravity differences. None of them are 100% accurate, and the difference gets worse the higher the gravity is. Supposedly the closest is the equation that's used by the Alternate setting of Brewer's Friend. The reason people don't use it is that it's hard to remember, and so it's not an equation that homebrewers tend to use since many prefer to do things on paper or in their heads. But there are specifically MANY different versions of the simple equation and they can differ wildly. Seems like BeerSmith is likely using one version of a simple equation. I don't know it as well, but I'd guess there's a setting somewhere to change that.