I don't know... a few days ago that would've been my strongest theory - there's a thought experiment in quantum physics called Shrödingers cat, where a cat in a sealed box is both alive and dead at the same time because reality is determined by how it is perceived- [And he can sense a shut up Reid coming in, so he'll just move to the point.]
Based on Einstein's theory of relativity, it would be easy to presume that we're dealing with separate dimension or worlds. One of the most famous theories within quantum physics is the many-worlds theory - that each action branches off in different dimensions where something else happened. For example, I'm drinking coffee but in a different dimension I picked tea.
Up until just recently I thought it was something like that... that us being both here and at home would be caused by separate dimensions or timelines and that previous versions of us would just be other paths. Now... I'm not so sure.