Why are simulations so different from eachother?

The physics seem like they change everytime I restart the simulation. Because of that I can't make a proper mortar cannon because sometimes the rocket just falls flat down, sometimes it goes well and sometimes it just flies straight in the air and all of this happens within the same ballast weight and rocket thrust.
You just gotta take into account that the game is running on Unity. This engine does not have constant physics, meaning that with the same initial conditions every time you run a simulation everything plays out in a different way.
Dunno if that answers your question.

Instead of getting mad at physics, use them to your advantage :p