As a basic explanation, a game with a frame limit will ultimately need to enforce that frame limit by asking the OS for the time. In an emulator, you can either try to estimate the amount of time that would have passed on the original hardware ("accurate"), or you can simply tell it the time which actually passed in reality on the system the emulator is running on (the host). If you do the latter, and also do high-level rather than cycle-accurate simulation, then there is no reason (and no way, really) for the emulated game to reproduce frame drops that occurred on the original hardware.