When sussing up upgrade chances I often look at both J levels on EF and the J seat map for the flight. There are often times that the two numbers are significantly different. My understanding was always that this either means there are ticketed J pax who have not yet chosen seats, or perhaps that a couple of seats are being blocked for pilot rest.

Is this pretty much the only reason you would see more empty seats than inventory for sale, or are there times when a flight could go to J zero and still have seats available for passengers? If so, are these seats put on the market closer to departure?