I asked my vet about this and she said that it maybe a habit. Cats from a pet store or a small cattery may want/like to sit in a litter box since they do not have their beds. Cats like to snug into something. Well, they do have their beds now! I also have air-con on most of the time. The thing is I have four cats. My first two cats did not sit in a litter box. When I got my 3rd and 4th (Ally and Jig Saw), Ally likes to sit in a litter box. Then the other three later followed. Well, it is still a mystery to me!