Can't really say, there can be a lot of reasons. I for one was a BIG tomboy as a kid and use to speak like that. I would even hit guys in shoulder, sit like a man and work with engines just to prove I can. I had a much older brother that I looked up to and because of it my interests were in a lot of "boyish" things like cars, competition and rock metal. eventually most of my friends were guys and I felt I had to be one of them to be excepted. Except, you know, better because I was curvy with a small waist and large breasts. I was also abused which gave me low self-esteem so I tried to change who I was in order to fit in with my group which was, again, a hole bunch of boys. The biggest abuse I endured was sexual abuse and I resented being pretty or a girl because my brother was never touched by our father and I began resenting being who I was because of it. Yet felt like I was only useful for sexual attributes. I thought I had to be tough and boyish in order to never be hurt again yet only felt I can only be loved for the wrong reasons. I grew up and I grew out of it as I became a wife and mother. lol But some habits are hard to break (and not necessary to break actually) like the things I'm interested in but I don't "act like a boy" anymore or even a girl, I'm just me.
The point of the story is maybe she has something to prove, to be cool or something. Maybe she thinks guys (you) would like her more if she can make you relate to her by being "one of the guys". Or maybe that's just how she learned to talk and she's faking the way she speaks to you in her sweet girly voice thinking 'THAT'S what guys like'. If it's not obviously fake maybe it's just her being casual and you're thinking too much into it. Could be nothing more then that's just the speech she picked up which is nothing wrong with that. Girls CAN say dude and curse, it's not owned by men. Even I still say dude occasionally unknowingly. There are just different reasons as to why she does it...as long as you get to know the real her it doesn't really matter, right? But I have a feeling it's not the way she talks that really bothers you. Does she seems like she's always putting on a mask and you're not getting to know the real her but instead only getting the person she's trying to act like in front of you? If so, maybe she has low self-esteem and you two should talk about the reason why?