This is something of debate that has been discreetly going on for years, but I can't help but suspect Mace Windu didn't really defeat Palpatine (in a combat sense). It's kind of strange to see Palpatine overwhelm Yoda in almost every way during their later duel, but in the case of Mace Windu, he conveniently goofed off when Anakin was arriving. It was also shown during his duel with Yoda, he has absolutely no problem with having no lightsaber. Now, notice how when Palpatine is launching Force Lightning at Mace Windu he's acting all weak and pitiful, but the minute Windu loses his hand, he bursts him with lightning so intense it lit up his skeleton? It kind of really makes me wonder.