Non-comprehensive necessities in GM

Hello Rabbi,
In many cases in the GEM, when there is a necessity, it is not fully clarified – they explain why in addition to A, B is needed, but not why in addition to B, A is needed. This is especially common in triple necessities, where it is rare for the GEM to cover all the possibilities.
What does this mean in the Rabbi’s opinion? It is clear from the very question of necessity that the duality that is not necessary disturbs the G.M., but it does not fully prove that it is indeed necessary.