When I select say 5 questions and add 3 random to an exercise, is it only supposed to pick from those 5 that I selected, or does it always pick 3 from that category that the question is in?

I'm using the 1.5.x series of Moodle, and after I added a bunch more questions to the category it was picking from all of them.  I can solve that by creating sub categories (which I did) but I'm just wondering if that is default behaviour, or if it's a bug.

Thanks.
Thomas