I'm still sure that in the distant past, they made a mistake with the names/fluff of the Kingfisher and Cauldron Born/Ebon Jag.
The Cauldron's fluff talks about how tough it is, when its not that well armoured, yes its better armoured than a hellbringer but so are most Citroen 2CV's so its not exactly a high bar to get over.
The Kingfisher's name does not fit the 'Mech either, and its fluff goes on about its firepower more than its survivability, when the Kingfisher in many configs is a giant middle finger because they have CT mounted weapons, usually things like PPC's and LPL's because screw you that's why. But for an assault, its undergunned by Clan standards, but it does use its weapons generally more rationally, but it is exceptionally tough and far more fitting of the fluff that talks about the Cauldron/Jag's toughness.
So i'm thinking that in the dim and distant past, they got the fluff for the two mixed up, it was missed, went to print and they then rolled with it.