This is indeed an interesting topic. However, it's hard to say how the PMI "ranks" difference sources of conflict. All they say is the following: "Conflict is inevitable in a project environment. Sources of conflict include scarce resources, scheduling priorities, and personal work styles." (Project Management Institute, A Guide to the Project Management Body of Knowledge, (PMBOK® Guide) – Fifth Edition, Project Management Institute Inc., 2013, Page 282.)
I think it's pretty in alignment with what Rita Mulcahy is saying in her book, PMP Exam Prep, Eight Edition, 2013. Base on my personal experience, I tend to agree with this order, with only one exception - sometimes personalities with dominated egos will prevail on top of everything else, so you will get a conflict in places and at times where you expect them the least.
As always, an answer to a particular question on the actual PMP exam will strongly depend on the context in which the question is worded. We always recommend that while taking their exam, prospective PMP aspirants will make reasonable assumptions based on all AVAILABLE information in the question and select the BEST answer among the choices given.
The reason I say communication is that most of the items in the list go awry and can result in conflict due to poor or ineffective communication and project managers generally spend 90% of their time on communication.