I think you are right that the PNM's choice takes priority. But I hate to think that PNM's are going to have to start thinking analytically when they pref! I stll think the fact that she was matched with her C choice tells me that she was not high on either A or B's lists. Also, in my experience, and depending on the strength of the chapter, those B lists are less reliably girls who pref that house #1. So, the placement is not as reliably cut and dried.
And, let's not forget the quota additions. Those always throw a monkey wrench in the works!
Bottom line - recruitment is a very strange animal, and trying to get a black and white answer to why something turned out like it did is almost impossible (particularly in the case of a PNM getting her third choice)