Extra attacks are odd birds within the current combat design - the idea seems to be that you’re multitasking so that these attacks become ”free” (like when you dual-wield), but it’s not obvious which attacks become free and when. For instance, there’s no particular reason why a human’s kick wouldn’t be an extra attack but a centaur’s is, nor is it clear why this attack multitasking works. Or why it takes a pretty large number of strike ranks to do one attack, but when you split, the next attack comes almost instantaneously.
RQG combat is a strange and frequently incoherent patchwork of effects and rules. It would be an interesting exercise to rebuild it from first principles.