There was a bug with summon before 3.0. 2.X : You summon your fairy, the fairy keep the stat of the actual weapon. If you switch weapon with a more powerful one (or less) with your fairy still there, the fairy keep her stat from the other weapon, not the new one. Remove and summon again the fairy will refresh her stats according to the new weapon.

Don't know if it's still the case in 3.0.