The idea that there are multiple people in the mind is likely a reflection of its malleability just as is the (typical) conception that there's a single person with a single concrete, tangible identity in there. The fact that different groups of humans plausibly claim to experience one or the other state of being likely tells us something about the flexibility of the substrate we're running on. Moreover plural self-conceptions seem pretty compatible with some well-regarded (while very speculative) theories of mind, e.g. Dennett's multiple drafts model.