IBU calculators are pretty much consistent and follow one of the established models: Tinseth, Rager, Garetz. There may be something in your recipe or set up that is affecting the calculations. If you export the recipe from BeerSmith as a .bsmx file (highlight recipe > click on 'file' > 'export selected' and a save window will pop up). Attach the recipe to message on this thread and we can look at it and make recommendations. Also, if you link to the same recipe in Brewer's Friend we may be able to tell you the differences which may account for the change in IBU.