It's because the Eliksni aren't true arthropods like we think, as life seems to much more complex on other worlds than Earth to just be fit into a mammalian, reptilian, avian, or insectoid type. It seems that, according to their features, they are a combination similarities to arthropod and reptilian, having both and endo and exo skeleton and reptilian like scales in some places with having tougher exo skeletal plates in others.