I think the last time she saw Mat was back in The Dragon Reborn, at which point Mat was still running away from any responsibility and could only be relied upon if you forced a promise out of him. This would be the same reason she misjudges his curiosity, his willpower, and his unwillingness to be coerced. Mat has grown as a character since he last saw Verin, and Verin failed to realise this. She outwitted herself.