why resultant-consciouness neither wholesome nor unwholesome ?

Because resultant-consciousness is resultant of wholesome-consciousness or unwholesome-consciousness.  They are grouped in dhammasaṇginī. Buddha use kamma to group consciousness (wholesome, unwholesome, resultant, neither kamma nor resultant).

But cetasika in abhidhammatthasaṇgaha are grouped by anuruddha (1000 B.E.). He use consciousness-accompaniment-ability of cetasika, in the sensuous spheres, to group cetasika.

You must separate author's method to learn each book. Buddha and Anuruddha thinking the same, but each book teaching by difference method. No one wrong.

anuruddha's method:

1. 13 cetasika that able to accompany with consciousnesses in every spheres.
2. 14 cetasika that able to accompany only with consciousnesses in immoral consciousness sphere (unwholesome-consciousnesses).
3. 25 cetasika that able to accompany only with beautiful sphere (wholesome+resultant+neither kamma nor resultant consciousnesses in kama-sphere, rupa-sphere, arupa-sphere, and wholesome+resultant consciousnesses in sphereless).

By Anuruddha's method, we can count cetasika unique. It is easier way to learn, but Anuruddha's method is vipassana less. Dhammasaṇginī method is vipassana full. We can memorize Dhammasaṇginī to do vipassana meditation, though we learn it some part. But in abhidhammatthasaṇgaha, we must memorize the whole of abhidhammatthasaṇgaha to do vipassana meditation.

Amazing Buddha! He really know how to teach for enlightenment.