Total guess here, but I bet that the Prince and the MC met when she went to the castle years ago, when she said she got tired and went in to the garden or w/e and it caused all of the flowers to bloom since he would have had the ring. However he does not know which girl it was which is why he has brought in all the noble girls in this age group to the castle so he can meet them and he can figure out which girl he met years ago. Aka she has basically been pegged as the next Queen for years, no one just knew it was her.