Sorry if this has been posted before, I've had a look around but I couldn't find anything...

I'm a little confused as to how the Top 15 works - I've had a couple of sifts that have made the list, have been younger than some of the other vids (I assume that there's some form of published 'age' limit), and have had enough votes to stay there, but which suddenly drop out. The Recoiless Nuclear Rifle one is a good example.

Is this just the way of the Sift?