Verin is the one sister I can think of who would have a reason to want to appear weaker than she is. She wants to be underestimated and carefully cultivated an image for herself that was bookish, distracted and harmless. Her strength level was higher than average, high enough that she'd be listened to in any group of Aes Sedai, but but not so high that it would mark her out. Perfect for the sister who wants to fly under the radar in both her Ajahs. But she only had a reason to do this - surely? - once she'd joined the Black, and she didn't do that until she'd been Aes Sedai for several decades. Her age is roughly 150-200, and she's only been a mole for 70 years. So by that time Verin would have long since have reached her full strength. I also don't see the parallel to Elayne.