Others didn't know, not even the members knew. Hyomin and Jiyeon go way back, they were childhood friends.

And for a period of time, a little more than friends. Until Jiyeon tells Hyomin that she dreamt of someone else. 

Jiyeon choose to chase after someone she couldn't even see, and everything for Hyomin became a mirage. A pipe dream. 

While others didn't know about the past, they saw things clear, only Hyomin and Jiyeon aren't aware of their own story.