>>3703320No real reason really. At first abyssals would have Oni and then Hime for the second phase (War Hime was used once for a third phase)
After that they introduced Water Oni which were similar to the previous girl of that ship type but somewhat different (Aircraft Carrier Water Oni/Seaport Water Oni)
Nowadays it's just all over the place really.
The -class abyssals take their names from this or at least I remember reading that a few years ago
https://en.wikipedia.org/wiki/Iroha 