because its presence in fandom is substantially lower than other pairings as a whole.

Overall presence/volume in fandom was what peskywhistpaw and I were going for when we began to write the list. Just because a ship doesn't have the volumes of fanfiction that Draco/Hermione has doesn't automatically make it rare; those that still have a considerable volume as compared to fandom as a whole can still be called not rare, even if as compared to another ship, its volume is much less.