Hi Doma,
This is actually a fairly well documented psychological phenomenon. Magicians depend upon it. It all has to do with how our brains wire themselves up when we are infants, and also with how little processing power we have when it comes to large amounts of stimuli. Basically, if you don't expect to see something in your field of view, it is harder to see it.
http://en.wikipedia.org/wiki/Change_blindness
Try this experiment, and you will experience it for yourself.
http://www.youtube.com/watch?v=vJG698U2Mvo