Why does cecily say she was engaged to “ernest” (algernon before she ever met him in person?

Cecily is known to be a sweet innocent girl and a romantic. She liked the idea of being romantically involved with Jack's "wicked brother" even though she hasn't meet the person. Although the idea of marrying a person named "Ernest" is also important to Cecily, she was more intrigued by the concept of wickedness, a rebellious romance. She even invented an elaborate romantic relationship with this Ernest all in her head supported with love letters with written by herself with fictional events, a ring and her diary entries.