Whatelse could it be, what you're saying makes no sense. Did they add an RNG on the mission saying after you complete 15 kills lets roll the dice to see what mission we give you? I highly doubt that. Kills that you are saved are still giving credit, so it is possible that one of those raiders killed more bosses to help out a friend or something and that increased their kills. It's hard to say but I can't think of anything other than kill count that would enable you to unlock one mission rather than the other so it makes no sense 2 people with absolute same progress getting different missions.
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That doesn't check out either cause someone earlier said they had 8 kills in mythic HM and recieved a heroic BRF cache when they leveled their followers to 660.