From what I have figured out, BeerSmith numbers the additives by the order of addition. When one or more ingredients have the same time or point of addition, it will then order them based upon the following priority: Fermentables, Miscellaneous, Hops. Within these groups, it will list ingredients by the amount of the addition from largest to smallest and then alphabetically by the name of the ingredient.
This ranking makes sense for the majority of brewers who are looking at adding a list of ingredients and then easily identifying the priority items based upon the weight/volume for those being added at the same time. If you have a number of ingredients being added at the same time, then it really does not matter if the recipe you are copying lists them from smallest weight to largest, or randomly as (in the case of grains) usually all be mixed together and added at once.