Are you on PC? Apparently there is/was(?) an exploit that allowed players (on PC) to level up their Followers to 65. I'm on Xbox One and have never encountered this. I've have howver seen threads here from players on PC who have fought other player's Forts (via Online Siege) who encountered level 65 Overlords, level 65 BG's to Warchiefs, etc etc.
That being said, enemy Captains in Sieges (Overlords/Warchiefs/BGs) can level up during the siege by killing their own allied or enemy Captains. In Overlord Fight Pits the 'enemy' can level up after kililng your guy, but if you're starting an Overlord FP and the intro shows him lvl =>61, then yeah someone is likely using that exploit/cheat.
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