Unfortunately, the correct answer is the one that doesn't make sense -- there is no logical reason for the difference in skill, with the exception of "there's something wrong in the game code" in regards to Valiant shot.
A little bit of extra info, Valiant shot also has a higher frequency being "dodged" by enemies compared to centaur's arrow. I advise not using putting it on auto-cast, instead only using it every 25s, or not at all.
You can, however, use valiant shot to increase your attack power and then resummon a centaur. This new centaur will be stronger then the previous one due to more attack, but again, aside from the attack boost the skill is not worth using imo.