Can't tell if it is a bug, fail design or just a mistake they made and have not fixed yet, but yes. A foul chocobo is way better than a fair chocobo, because, while the fair chocobo may be faster in sunshine, all his stats are CRUSHED by rain. You don't see much of a difference at foul chocobos, they are not really better when it is raining, but they are also not worse, when the sun shines.