I have never ever been in a game in which a skeleton was created to replace another one that died. This is because, as stated before, the event that triggers graveyards' ability is the death of a non-skeleton creature. If this check wasn't specific but instead checked for anything that qualified as a creature, then a player could simply play 3 graveyards and let one skeleton die, generating 3 to take its place. That is clearly an overpowered effect, and the reason why a Graveyard/Boneyard checks specifically for non-skeletons.