Leader gets less XP than the rest of the party

Or rather, Leader gets the normal amount and everyone else gets extra XP. I've started writing down before and after experience levels and it turns out after each mission Leader gets the expected amount of XP (from kills, containers looted and all extras that pop up like "scavenged all containers") and all other survivors get 1.2 to 1.5 times that amout (they all get the same amount). No XP books or gear is equipped. The issue only happens when zombies are involved (missions and attacks on compound). Everyone (leader included) gets the same XP from achievements, compound upgrades or tasks. For example: Lv14 mission, the report at the end of the mission shows 1889XP gained (1021 from kills + 868 from containers). Add 272XP from extras during the mission (you have to note them down on the fly). So theoretically, total XP from the misison is 2161XP. Leader goes from 69929 to 72090 which is exactly 2161XP gained. The other 4 survivors involved got 4072XP each. Another one: attack on compound. The report at the end says +532XP, which is exactky what the leader gets. All other survivors got +847XP. At this point I'm tired of running solo missions so Leader can catch up.