None of the planes I see are shown on the radar

Greetings. I just discovered this site. When I first saw it I thought, "wow that's cool". But then I started looking outside my window, at the planes passing by. I have seen maybe 10-20 high altitude planes today fly by. But noone - NOT A SINGLE ONE - was shown on the flight radar. They are simply not there.

Now I just wonder, is this program a hoax or what? Where does the program get the plane coordinate information from? I just think it seems very unlikely that no planes passing by here would be listed. I live in northern Europe btw.