From an old message by Guthix on the Runescape main site: He described drops as being on a roulette wheel. There are two wheels, one for determining what drop you get, and there is a portion for rare items. If you got the rare item one, then it would spin another wheel determining what rare item you got. The RoW would increase chances of getting a better rare item on the second wheel. We don't, however, know what items are "rare" or not, so it's a matter of preference.