Why is it a Kasha? We actually do Pasken that it is Reshus, although it was accepted as a Chov and we may not drop it.
As for the Chiluk, perhaps the difference may lie in the reason for Paskenning like Rava or Rebbi Yochanan. Rava is considered Sinai and we assume he is coming from someplace. When we already have the Shittos layed out Rava and Abaye are both equal. The reason for being Machria like Rebbi Yochanan is not because of a specific strength. Ad Kan my speculation.