In short, simulation is only a simulation which reflects the efforts we make to model the physical world. Many varieties of modeling approach exist, ex mathematical behavioral, transaction.....etc. That is to say that we can only make it approximately correct in the sense we can aim the target, but never be exact.
Not unusual, with the constraints of our tools such as computational power, it just takes too long, in some case even in the extent of our life time, to get the end results of some simulation case. Imaging a high definition video source, how long the simulation will take to proceed only one second real time for a codec design in a RTL level. In such case, engineer has to do some tweaks to complete the task in a reasonable time.
Another example is about the initial settings of system variable. There was a case when engineer was complaining that he needed one day long to do a simulation run for a small design, actually a oscillator. Without appropriate initial value, the simulator was trying to estimate a initial condition which was happening to be half or middle point of the equilibrium state. Oscillator like a seesaw, it would stay at its balanced state if no external force in the sense of mathematics. So the solution was to give a initial value to make the seesaw starting from a unbalanced state, then the simulation run completed in some minutes.
A joke here, but real case, an engineer was confused why the waveforms shown in the scope just didn't look like those in simulation software. As an engineer we always need a sense to tell simulated or logical case from physical real world.