Okay, so the Seven Genders seem to be kind of associated with planets - at least, the spirits of Saturn and Jupiter each exemplify a gender - but also, as you note, there are the wrong number, and also Mercury (like Mars) is simply masculine, so there's some kind of relationship but not any kind of neat one-on-one correlation that might give us a clue about what the unnamed three are like. Is there a Pluto gender and is it the gender of being very small and cold and distant? Who can say! A mystery!

To be fair the Seven Genders thing appears in one chapter and then never again, so it's clearly kind of a side issue, but all the same I can't believe he told us there are seven genders and then only gave us four.