The guy in front standing is “finished” eating and is in limbo state (semi stuck and between commands). The guy behind is sitting on a stool eating normally.
Colonists have difficulties sitting eating at a table and prefer a standalone chair instead. Object collision are “everywhere”.
Also. To get them to use the special lunch table try to remove one chair at each side. Although they still may not use it, for above reasons and that they will use the closest seating, or closest to their next objective.
You are right, they prefer single stools. This one is even farther away than the one in the room above.
Suddenly I can’t upload pictures or links of them here . Strange, yesterday I could.