Why did this guy look surprised when I said hi back?

I switched into this new class and my head was down and he couldn't see my face. He sits in front of me and said hi to me so I looked up and said hey and smiled at him. He looked stunned for a few seconds like surprised and his eyes widened and after a few seconds of that he smiled back lol. Why did he do this?
Why did this guy look surprised when I said hi back?
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