Ayanami originally was not and Z23 was for CN, but for JP servers it makes sense that they'd want one of their ships to be a starter choice, so they swapped Ayanami/Z23 so now the German ship is the one that needs some work before you can unlock her.
Thus, for JP artists at least, it makes sense that the above trio is commonly drawn together while Z23 is absent.
Ayanami originally was not and Z23 was for CN, but for JP servers it makes sense that they'd want one of their ships to be a starter choice, so they swapped Ayanami/Z23 so now the German ship is the one that needs some work before you can unlock her.
Thus, for JP artists at least, it makes sense that the above trio is commonly drawn together while Z23 is absent.