I wouldn't consider the ratings very much, neither are EN tested, so just a guess on the owners part.
What is the relative surface area of each? since they have almost the same amount of down, they will be equally warm if the they are sized the same and the other features are the same.
I'd imagine a draft collar really helps. Especially if you move around at night. It helps prevent both drafts and compression of the down around your shoulder by over tightening the neck. I've loved one on the mummy bags I've used in winter.
The footbox is warmer on the Katabathic as well. Mummy style trapezoid design means no dead air, no drafts and no compression. In contrast, the fully opening bottom of the RevX is very versatile for warmer weather, but creates a minuscule gap, as well as a significant area with low loft. Also, depending on the width, it is very wide in the foot area, increasing dead airspace. This is less of an issue if you are using very warm down slippers.
Finally the Katabathic has a differential cut, helping to prevent cold spots from compression.
In short I think the Katabathic will be quite a bit warmer and easier to use in very cold weather, especially if you don't use a bivy, while the RevelationX will be more versatile for use in higher temps.
Als, if you want lighter, and don't mind paying for it, you can of course get the regular Revelation (non X) and save some weight.