It's simple. If the MC is on Jill's path, she doesn't want to give the MC a reason to be jealous with the banana costume. If the MC is not on Jill's way, she wants to show him what he is missing with the bee costume.
Elena is actually the antithesis of that. If you then read through what the MC thinks about the costumes he has chosen for the competition, the thought comes to me, so the girls want to know what the MC is into, what his girl should wear.
That's why I always choose Riona from now on. The MC describes her costume as Cute, Hot and funny.