Generally speaking, when running the parallel multi-core computation analysis engine (compute), the results will be slightly different for each run. The reason is that the order of operations are not the same for the simulations due to the use of multiple cores in your CPU. However, if you run the model with the non-parallel version of the compute engine, the results will be the same, if you run the model on the same machine (it will not be the same if you run the same model on different machines).
To always use the non-parallel version, go to the File menu and choose the Preferences option. Turn off the Use multi-core processing for compute engine option.
RS2 Preferences dialog
You can also run the non-parallel compute manualy through the start menu, by selecting the Sequential compute from the Utilities menu in the RS2 start menu.
Note that the non-parallel version can be 2-3 times slower than the parallel version.