Well to start, your initial comparison was severely flawed, as if the Ogre had a 40 Str, the charasmatic guy, could have had a 38 Cha if he was say Trueborn for example and concentrating on being persuasive. The point though is the character is legendary, it doesn't really matter for what, but people respect them enough because of that to listen when they speak. Really I don't know why Diplomacy isn't in each legendary skill group for that reason, but I guess they wanted some diversity.
Anyway, each path has a strange skill connection or two...likely on purpose. Mighty (Diplomacy), Swift (Intimidate), Unyielding (Survival--when related to tracking), Learned (Climb, Hide), Wise (Hide, Tumble), Persuasive (Concentration, Heal).