Wanna know the answer?
Its random. Wanna know how I got to the answer? Logic.
Ingame Lucario that do not have DW abilities have either Inner Focus or Steadfast. Riolu's only ability, not counting DW abilities, is Inner Focus. What is the point of giving a second ability to Lucario if it stayed the same? Easy. Because Riolu has a chance to evolve into either of them.
A Pokémon that evolves and has a certain Ability in its pre-evolution (dual-Ability Pokémon only) but evolves into a dual-Ability Pokémon upon evolution (such as Poochyena with Run Away or Quick Feet evolving into Mightyena with Intimidate or Quick Feet) will keep its Ability from its pre-evolution upon evolving (so a Poochyena with Quick Feet in Generation IV would keep its Ability upon evolving into a Mightyena, while one with Run Away would evolve into a Mightyena with Intimidate instead). If the Pokémon is a single-Ability Pokémon pre-evolution, then it will have a 50% chance of getting either Ability if its evolution is a dual-Ability Pokémon (dependent on IVs).
50% chance. Depends on IV's.