Why & How do you explain that we humans experience what's called 'Deja Vu'?

Depends on your outlook on life i guess... If your more spiritual you could look into the idea that de ja vu is where you may have had a similar experience in a past life. But the most logicial idea is that your brain processes a scene faster than your eyes do, hence when you eventually take in what you are doing it feels as if your seeing it or responding to it for a second time.