if you use some random values (e.g. availability, random distributed process times...), than it may be, that you get different results. You can check in your model the attribute: randomSeed of the corresponding elements. If there are differences (e.g. different orders by inserting into the frame --> other randomSeed - value), than this elements create different random-numbers and so the results are different.
Another reason can be the setting: tools - increment variant at reset in the EventController. If this is activated and you use random numbers, than each run produces different results.