When a resource pack has an entity.generic.small_fall or entity.generic.big_fall sound added via sounds.json that isn't located and named like the original one that plays in-game ("damage/fallsmall" or "damage/fallbig"), it doesn't have a chance of playing instead of the one titled like the original audio file. That isn't the case when using the "/playsound" command, which behaves properly, being able to play all sounds in the category with the same chance.