Mercenaries (Assassin's Creed: Odyssey)

Started playing the game a little and don't understand the system of mercenaries.

As I thought, it should work like this: someone from the mercenaries started for you to hunt, you meet him kill, and he never poyavlyaetsa in the game, and so with all.

But in fact, for some reason they respawn after some time again you can meet the same mercenary.

I think it's the wrong system, or something I do not understand.
Can anyone explain what's the point?