How???

I received this image and i would like to know if theres somebody who can explain to me whats happening in it.........first time, how many guys can you count there? then, the image moves only a little bit and how many guys are there after that? why the number of guys changes if only the image moved a bit???

please could somebody very smart explain this to me???

because ive already given up of trying to find it out lol :-S