Calculation Differences Between Programs

Ok, I downloaded the trial versions of Beersmith, Beer Tools, Promash, and even the free Brew Target. I entered the exact same recipe into all of the programs and verified that the potential SG of every fermentable was the same. I also verified the variables that I could, such as times. I really like Beersmith (but wish it had a few features of Beer tools), but of a recipe I did, it came up a full .0100 off wheras Beer tools was dead on. What I don't understand, is why all of the programs came up with a different OG??? Any ideas? Any ideas which one is most accurate?

I suppose it depends on what specs they're using on each malt and from which company. That's why I like BeerTools and the ability to choose which manufacturer you bought your malt from. I've never used the other three though, so they may be the same.