I think that it is probably related with the difficulty level that is measured by the number of stars in Cyclopedia (from 1 to 5). I just don't know how many stars for each category. However, that are creatures marked as common, like the Ravenous Lava Lurker, that will never appear in the random selection, only if you pay 5 cards to select. I saw the list of excluded monster here in TibiaQA.