because there is no time. time is an illusion, as there is only ever "now", and so as an illusion, it can be cracked and seen for what it really is. we can therefore look over the entire river of perception in the now moment, what has already flowed through our awareness and what is coming up, and choose which part to put our attention on.
more of a shamanic take on your question than a scientific one...but that's how i roll :) after all, Einstein showed us that all matter is energy, and energy can neither be created nor destroyed. so while it might not be possible for "matter" to jump back in "time", energy (the essence of our material form) is ever present, and much more fluid in its existance...and in turn much broader in it's awareness and what it could possibly be aware of. like what has happened, and what will be.
I'm sure my answer is about to be devoured by the know-mores of hp, but I enjoyed answering your question this morning. thanks for posting. be well.