The reason why certain pokemon have better stats than it is because what we view as “Arceus” is just mearly an avatar* of the real Arceus. *Avatar: a manifestation of a deity in a bodily form.* And Arceus only put a fraction of it's power because of that “Arceus” only has a base stat total of 720. If the real Arceus were to show itself nothing would be able to comprehend what it is. The only things that could comprehend it would be Dialga, Palkia, Giratina.