Have you never played a PC game before where there are certain areas where you reach 70FPS, others where it runs at a perfect 60FPS and other areas again where it suddenly drops to 20 FPS where the requirements are a little higher?....well the same sorta thing applies in emulation, some games have certain areas which require more power than others to render, power which when it's being emulated your system may not have enough of..or sometimes too much...now the impact is reduced when you use settings other than realtime because those other settings increase the amount of frames being skipped that's why you can achieve percentages even higher than fullspeed in certain areas.
So to give you a short answer short, you can't get fullspeed all the time because you either don't have the processing power or the emu is not programmed efficiently enough to harness all the power it needs to emulate that area fullspeed.