Maybe they don't have the slots linked to the category pages in real time...
Let's say they upload a new game to the action category? That wouldn't neccessarily mean that it is placed at the slot in the left top corner of the 'New Games' section on the homepage.
I think they manually copy/paste the thumbnail and link to the new game into that section. That would be explaining why one would be empty for a second and then filled again! =)
But we won't absolutely know for sure untill an admin (is willing to) answer(s) this mystery...