Whenever this happens, it means that some piece of the algorithm involved probably required starting values, usually for a search of some kind. In that case, a random starting point is probably generated, ergo you will get potentially a slightly different result based on where the search originated. Not all such searches will always end in the same solution, if multiple solutions exist. Or the solution might be subtly different, but still within tolerances that were deemed acceptable.
While I don't claim to have looked up the specifics of the algorithm involved, I have no doubt that a varying result is due to a random start somewhere inside that algorithm.
