That is because One Man Army is a feat that works just as well with AND without a party.
How would you know?
You can't take it and level up afterwards in the EA version of the game.
Taking it on a solo character gives +65% Vitality and +2 AP and nothing else.
You would need to level up to 9 to see any change in attribute and skill points per level. But there's not enough xp available to do that.
It says it gives a scaling bonus based on party size. Well have you tested taking it with a party 1, party of 2, and party of 3 to support your statement that it works just as well?
Show the number please.
I tested it on a solo character and saw the +2 AP and +65% Vitality. Couldn't ask anyone else to join to see how it affects the numbers because nobody else was alive. That was the only way I could reach level 8.