Your study is your study, and you might be very right on your conclusion. If that doesnot match with mine, doesnot mean I am wrong or you are wrong. We may be right on our own observations.
On general cases, girls get into a relationship after lots of thoughts, judgment and logic. Its love, which is supposed to be from heart, but as I have seen, on love or lust issues, males are much more reckless when females are more calculative. If you say a girl falls in love and keep a back up plan, I dont agree much. When a girl fall in love, she falls in love with her whole mind and heart, there is no space for a second person unless there is a mistake or obligation.
Suppose a girl chose you and said that you are the boyfriend. A girl fall in love with the person and try best to like what he likes and adjust in her own way as much as possible. Now if she finds later that you are not the right one, she will try to find a suitable one that matches her wish, once she settles on that, she would go for a breakup. I always have seen girls can love only one person at a time, unless there is an exception. Exception means she loves one person for real love, and the guy is bad on sack, so she falls for someone who can meet her physical needs. These types of girls are not significant in number.
For clearing your confusion, I would ask you to sit with your girlfriend, try to understand whether she is in a transition of changing her mind about you. If the person you didnot match with her expectation on you, get ready for a break up soon. We can love someone but cant force anyone to love us. Before you get a heart break, its better to be ready. Dont do spying on her whom she is talking or her mobile, that is violating one's privacy.
On your shoe, rather than looking at other girls why they keep a back up option, I would look at myself if I am doing the right thing as her expectation or not. You sure are doing something wrong, that you also know, whom she is talking how close they are, why she hides her mobile, these are all sign of suspicious mind. A girl doesnot like a man with suspicious mind. They want want to trust unconditionally and be trusted the same way. If you still have time, change yourself a bit, else I have to say you are less in love with her, more in love with yourself, you love the feeling of being in love and other people recognise that you have a girlfriend.