Why does this girl smile, but doesn't maintain eye contact?

I am trying to improve my social skill because all my life I have been pretty introverted. I find it pretty easy to talk to other guys, but I find it a bit awkward talking 1 on 1 with the girls. So, I started talking to this girl in class. Which is unlike me because I often wait for a girl to initiate a conversation with me. Unless we need to do a project together. Usually when she talks with other guys she has no problem with maintaining eye contact. When her and I talk she avoids looking directly into my eyes. We were sitting down next to each other. As we were sitting down and having a small conversation. I saw her smiling, but she was looking straight ahead in front of her. I don't want to personally ask her why. I think that would make things really awkward than it already is. Any input would be greatly appreciated! You can ask me questions if you need more detail.

Why does this girl smile, but doesn't maintain eye contact?
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