How is Olivia's aside at the end of (act 3 scenes 1) ironic?

This is of course a play full of people pretending to be other people. Viola warns Olivia as best she can, telling Olivia that "I am not what I am," though Olivia does not guess at the statement's real meaning . Of course she is unsuccessful, and Viola leaves ‹but not without an entreaty to return.