I'd say they all "got it wrong" insofar as it's clear they don't seem to grasp the idea that the two things are the same, but are instead providing a response _

I'd say they all "got it wrong" insofar as it's clear they don't seem to grasp the idea that the two things are the same, but are instead providing a response _as if_ one were comparing feathers to some other material. Or, rather, perhaps it would make more sense to say they "got it different" than a human probably would, who might clarify that you intended to compare a thing to itself or simply say "They're identical, so they weigh the same" or something of that nature. Instead, the models all explain volume, density, etc which are applicable if you're comparing feathers to lead or steel (as is common in their training data, I'm sure), but doesn't make as much sense in this specific question. Yes, yes, I realize different bundles of 1kg of feathers could have different volume and density. But it seems clear that's not the intent of what the models are communicating.