Ah, thanks! That indeed appears to be what is happening here. I didn't realize that one fare class could be referencing only one leg of an itinerary while another is taking the whole itinerary into account.
I'm searching for LAX-BWI. It's the LAX-ORD leg of that itinerary where PN > P. When searching on LAX-ORD only P = PN = 0, which makes sense. ORD-BWI has P9 PN6 P0. But does that make sense when combined on one itinerary?