You may consider a nin/tai mystery/standard attack actually as if you were landing two separate attacks at the same time, one full ninjutsu and one full taijutsu. The first part is based only on your ninjutsu and gets reduced by the enemy resistance, the second part is based only on your taijutsu and gets reduced by the enemy defense.

That's because is a special trait, like tendo immunity to ninjutsu and is specifically said 'to pure ninjutsu...'.

I was just explaining it damagewise to make it easier for them to understand.