If they are on the UWP for the sector, they are Moot members, and High Nobles. There are just more of them than there are subsectors. This usually means that there is some special district, issue, or potential issue that warrants high level Imperial oversight. Proximity to the Julian and/or Vargr border might account for some of them, similar to what you see in the Marches.
Nobles without a "place" aren't High Nobles. They're the Honor Nobility, aka the Emperor's Own Labor Pool.
My next step is to see if Lishun is unusual in this regard. I know some of the other core sectors are lower density, but I haven't looked at all of them yet.