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My understanding was that any active search words was eligible for display regardless of quality, but Google would estimate the bid that would be required to land on the first page. Obviously, this isn't always the case.
I'll try to identify some terms in my account that are experiencing something similar and see if I can get them to show by bumping the bid.
I'll be back...
Here's that thread:
why is the first page bid high when noone is bidding?