Yes, good plan to talk about it.
I agree that most likely scenario is that she skimmed it, and dwelt on that one part for whatever reason, so she isn't lying to you.
If you can talk about it, it might help you to look at how difficult it is to trust her, or believe her, and may lead to a really helpful discussion in general. Cos maybe the issue here isn't just the letter, but a bigger issue of trust?