Why is she sometimes nervous, sometimes not?

This girl and I have been up down back and forth for a year now. She gives me all the signs in the world usually she's pretty nervous fidgety and jumpy around me. The thing is she will occasionally be cool as a cucumber and seem not nervous at all. It just throws me for a loop and I second guess if she's digging me when she acts like that. How does she do it and why?
Why is she sometimes nervous, sometimes not?
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