I've recently attempted a retribution build and notice a few things with the Bull Horns ability.
When I had less than maximum attribute points invested in Retribution and I used Bull Horns with Lone Wolf Trait I was receiving +2 retribution instead of +1.
When I had maximum points invested in Retribution (5 +5 from lone wolf) and used Bull Horns, I would have no change in retribution. Without bull horns showed (+5 from lone wolf), with bull horns showed (+4 from lone wolf +1 form bullhorns).