Why is she avoiding the boyfriend word in front of me?

Anonymous
There this girl I was interested in and we hit it off really well but I found out she had a boyfriend (she doesn't know I found this out) but she has never mentioned him to me and she never talks about him to any of her friends when we hangout in front of me, one girl recently mentioned her boyfriend only by name though but she made a point that this guy just a friend of hers, in the past she referred to this guy as her brother as well, Recently I've tried to keep a slightly lower profile around her because once I found out she had a boyfriend my interest in her began to fade quite rapidly. The fact she blatantly covering it up in front of me makes me suspicious though? and its made me question why she doing this? also found out she been with this guy for least a couple of years or more and had some rough times in their first year of their relationship however I was so dam sure the early days she was interested in me, we still get on well but we are both quite shy people. Usually its other way round with girls in relationship they always say "MY BOYFRIEND" etc in some respects its good because you know where you stand straight away
Why is she avoiding the boyfriend word in front of me?
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