Why does this girl stare at me but ignore me?

There is a girl I have been seeing around my university for a few months now that is always staring at me but will avoid me and pretend to not see me when I'm close to her. There has been several times I have looked up and noticed her looking right at me from across the road. She also often walks by the campus gym when I'm in there and will look at me through the window. She never really smiles and just looks at me with a blank expression. We don't know each other and have never actually spoken. One day I tried to be nice and waved at her as I passed her and she smiled and said hey back but then went back to avoiding contact the following days.

Why does this girl stare at me but ignore me?
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