I'm no expert on this, just my experience as a prior frequent flyer, beneficiary of the occasional buddy pass, and a month or so commuting once entering this job, so take it for what its worth, but some people booking tix can't get assigned a seat (they have a certain number held I think, especially the "premium" seats so if a FF wants to book a ticket there are still "good" seats left), or they are out of econ seats and the pax don't want to pay for econ+, so they get no seat assignment until people get upgraded or move to econ+. When I was silver elite I think I could get upgraded or econ+ free 24 hours out or something like that, so regular econ seats become available. So any seat that doesn't have a seat assignment shows open on the seatmap (so you can theoretically book it), but there are more booked than have seats (usually), and they will start getting seats as they check in and people do the last minute shuffle trying to stake out real estate.

Best bet is to use auth/cap/booked until within 24 hours of departure, then cross check both, while looking at the nonrev stby list, from my experience.