I confused about Training Missions, specifically which Recruits get XP.

First Training Mission of the day I choose Comprehensive. Only my initial four recruits can get XP, regardless of who is selected in the Team via the Sergeant. Second Comprehensive of the day only my recruits 5 and 6 are getting XP if I select Comprehensive. If I select one of the other missions only recruit 5 is getting XP.

Help me understand this please and whether there is anything I can do to help decide which recruits get XP for Training Missions.