Take an acacia leaf block and a dark oak leaf block from the creative inventory, and place them on the ground.
Then, delete the items from your inventory, and try middle- clicking one of the leaf blocks to get that item into your inventory. (in creative)
If you middle click the DARK OAK, you get 1 acacia leaf block in your inventory. If you middle click the ACACIA, you get 0 acacia leaf blocks in your inventory. Literally, a stack with the number 0 on it.