Thank you both, what I was missing was that if I happened to have selected an officer that was too _high_ a level for the role, the role wouldn't show up on the list.
(I now wish I knew how Steve used this screen because, for me, it feels far more intuitive for the default to be a complete list of open positions and selecting one filters the list of officers than the default being a complete list of officers with the open positions, which are divided up behind a List Box, getting filtered.)