she has a way of knowing things without really thinking about them, hence able to hide things in her memory even from enchantment.

Aha, and maybe this is even the same as Polly's hero-gift of knowing things, as Tom points out when she first meets the quartet? I hadn't made that connection before -- maybe gifts run in families.

(Sorry for reply-stalking all your comments here, but I'm having a lot of fun thinking about all this.)