I've seen this before when more than one page is relevant for a query but the top one gets penalized. Pages can rank without any external backlinks, especially if they are supported by internal links.
What is new in this situation is that you have three URLs on page 5. That seems to be related to the Google changes we're discussing in a few other threads, albeit for page 1. The "host crowding" filter at Google is going through some changes and apparent testing.