Is it possible that she isn't fully comfortable with the concepts? In my experience (as both a learner and as a teacher of my kids), I've noticed that basic mistakes show up when the student is still having to work at understanding what a problem is asking and at setting up a solution. I've seen this in my sons, my daughter, and myself. I suspect that the problem happens because the mind is focused on two big things: concept and calculation. The horsepower that's required to think about the concept diminishes the ability to calculate.
I've also seen that students can give an impression of understanding something when they don't fully get it. In my experience, this situation is common, and I wonder if it's even a normal part of the learning process. I've observed it not only in my kids (and myself), but in students I taught at a community college. They seem to understand something when I ask a straightforward question, but falter when I ask a question that comes from a different direction --- even if the second question is a logical extension of the first idea.
