Why is this girl surprised every time I greet or say goodbye to them?

I know this girl, I'm very much in love with them. They don't feel the same way, it's pretty obvious. So, I'm not looking for "They like you back". I know that this is the case.

However, the other day, I said hi to them and they just seemed a little surprised. One day, when I said bye to them, they were surprised. They have this facial expression of just surprise.

Why is that? Any reason to it?
Why is this girl surprised every time I greet or say goodbye to them?
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