Drinkers lie about how much they drink, because initially they know it has gotten away from them.
I agree with Naive, in that she may not be being honest about her consumption...
I also agree with Isaiah, in that it kind of does not matter. If you are not getting what you need, and if she is someone that you are wondering about in terms of why she is not emotionally available, then it does not matter why.
Ask yourself this: If you found out that she had a drinking problem, and that she was in bad shape, out of control, what would that change?
Would you then feel like her behavior is acceptable, excusable, or more tolerable?
Coming from experience, let me say that alcohol was definitely A REASON for my being treated badly by my A BF, but it was no good excuse.
He is sober, now, and with that comes the dawning, or re emergence of more and complex issues about WHY he drank so much. He is emotionally, psychologically unstable. We have a child of 5 years, with a total of 9 years invested. HE still acts like an ***hole many times, and that cannot be explained away.
I think you need to ask what you are getting from the situation, and at the risk of sounding cliched, try to remember that, even though you are going through pain and transition, there are so many other fish in the sea.
Good luck to you.