Companions do use default stats. Cleric domains were also given different default stats, with Trickery getting the short end of the stick since they couldn't decide if it should be Str or Dex. So Shadowheart has 13 in both.
I can't believe the game has been in development for 6 years and they couldn't find the time to give companions appropriate stats. No one ever questioned that maybe the most beefy character in the game should have more than 10 Strength.
