I'm trying to think of reasons why this would be so.
Start with the fact there were around 1000 sisters in TV, several of whom are out of the Tower often. It's common for Aes Sedai not to know a whole bunch of sisters, especially women they're not of an age with (and thus have not been novice/accepted with or under) and who aren't of their Ajah.
What Terez meant is Sister X isn't trying to hide that she's not reached her full potential, but this doesn't mean all AS have noticed it. Of course women who interact with her would nearly all have noticed this, while women who barely or not at all know her might not. It's also highly possible there are many sisters who have not really crossed path with her - Moiraine had seen some of the women in the Fal Dara embassy for the first time. Moiraine is out of the Tower a lot of time, but she isn't the only one in that case.