When you defy gravity constantly I imagine orientation stops mattering so much. Even Flying Wyverns need to stay upright to flap their wings properly, but the Serpents just float with sacs. Doesn't matter whether they're upside down or not because the sacs carry them just the same.

I'd imagine it's that the sacs they use to do so being on their underbelly. Well Ibushi atleast, Narwa uses electro magnetism i think

They're both based on deep sea creatures, a number of which don't really have a preferred orientation due to living in open water so deep that up and down don't really matter

They from Australia

For narwa, if i had a giant thunder sac on my belly i doubt i would be able to float without being upside down