Why does this girl act this way towards me?

I don't get it. She is a nice and quiet girl, the shy type but friendly when she gets to know people. She was this way with me when we met but little by little she started acting all sarcastic, making fun of me and arguing (its all done playfully). Its kinda shocking to see her be all nice and bubbly with people and as soon as she sees me, she'll start throwing little comments, being a smartass or sometimes she'll bait me by starting a nice conversation and THEN start throwing sarcastic remarks.

We do it to each other and frankly, we don't do it to anybody else. We are both friendly people but when we are talking to each other its like a whole different dynamic and people can tell there's just something different when we are together. Some of my friends tell me that she may have a thing for me but she acts so difficult at times that we sometimes get into stupid little arguments that I just don't see it. As much as we argue we always make up and start talking after a few days.

Why does she act so differently with me? Is that nice and sweet personality just out of my reach?