I maintain that it was one of Vandene or Adeleas. One of them held themselves herself under her full potential so that they would be able to switch/impersonate each other at need. For sneaky Aes Sedai-ish reasons.
Wouldn't work since Aes Sedai can sense both current strength and ultimate potential in another woman. If one of them held herself back it woul more likely be so that the other wasn't forced to defer to her and they could stay equals