Other mistake: When Lucy tosses the umbrella off the balcony during the Mexican serenade, Ricky somehow doesn't appear to hear it hitting the floor during the pause and keeps on singing. (00:23:30)
Continuity mistake: At the end of Ricky's song "Lady of Spain" when he sings the verse "all that my eyes are revealing, Lady of Spain", in this wideshot we see Lucy in front of the balcony doors facing forward raising her arms, then in the next side closeup Lucy's still facing forward and a flag has suddenly appeared behind her on the balcony door. (00:21:50)
Answer: There is no reason. It's a just a silly plot device, typical of the era. Women characters were often portrayed as making uninformed assumptions or decisions.
raywest ★