confused about the friendship hierarchy

This won't be about RC, Key, Econ Guy, or even Mr. C. Instead it's going to be about this girl I work with. We came into our grad program together. We worked together quite a bit on our overlapping classes first year; we randomly got girl brunches every now and then, and I was always on her party list, even when it got dwindled down to just the VIPs.

Except deep down I think we both have major issues with each other. I think I'm one of those girls who can get competitive with other girls on just about anything (school, boys, # of shoes we own, the ideal color/size of post-it notes), you know, trivial catty stuff. BUT, I don't do this unless it's initiated on me first. When I feel like another girl is being unnecessarily competitive with me, I get an extra dose of I-can't-let-go-of-my-pride, and I refuse to back down until I show the other girl just who knows best.

This girl brings out this competitiveness in me because she's ultra-competitive. Beyond competitive, she's sneaky. She would find all kinds of loopholes and borderline-shady resources to make completing her homework easier, and when asked about it, she claims them as her own work and is reluctant to share the knowledge. At some point after I picked this up, I tried to avoid her in general 1) because I had issues with her moral standards, and 2) I didn't want to feel the stress to compete.

It doesn't help that she's blonde, cute, and carries herself quite well, so she attracts a lot of men. And she is one of those girls who will lead a guy on for the attention, even when she otherwise has a boyfriend. So that was another reason I stayed away from her; I didn't feel like rolling my eyes whenever I saw her talking to a boy knowing full well that she has a 1-year boyfriend at home.

Okay, so back to the point. Every year toward the end of March, our department has a retreat with all the faculty and students. Some 150 people go off to a hotel outside the city for 3 days and talk about our work and do some socializing. They put students two to a hotel room, and we can request roommates. The past two years, I stressed about finding a roommate, trying to figure out who I can ask who would not want to ask someone else and who may not already have a roommate. This year, I decided all that was too complicated. I really couldn't care less who I end up rooming with, so I just didn't bother finding a roommate.

So today, this girl came up to me to ask if I had a roommate yet. I said no, and she asked me to be her roommate. I said sure, no problem, and she seemed really happy and ran down the hall to tell the admin organizing the retreat about her new roommate situation.

This got me thinking, how did she ever come to the conclusion to ask me to be her roommate? Going down her hierarchy of people she could room with, I'm about as far down the list as someone can get. How did she exhaust all her previous options to ask me? She's dating someone in the department; I would think he would be her first choice (90% of student couples room together). Then, she has a slew of girlfriends she can ask, one girl in particular who she actually lives with.

So I'm confused about her friendship hierarchy, not to mention how the retreat will actually go with her as my roommate.