So I hit up
GW:
DE on Facebook to confirm my little theory about the reason things didn't change with the following message:
Just a little design question about Codex: Inquisition that's popped up and I was thinking you guys might be able to shed some light one: is the reason that the Inquisitorial stuff mostly stayed the same because it's in Codec: Grey Knights?
Also, when Codex: Grey Knights gets an eventual update, does this mean that the Inquisition book will see an update on anything that's shared between the books?
Here's the response:
Games Workshop: Digital Editions Hi [Zion],
We try not to have wildly different rules for the same units in different places. The inquisitors in Grey Knights are still inquisitors, and your opponent should be able to know at a glance what they're up against when you put your models down on the table. (that's why things like Rhinos, chimeras and Tactical Marines have very similar rules wherever you find them).
Of course, one of the advantages of the digital format, is that you can change things much more easily, and you may well see changes to some digital codexes over time to match new print releases or new miniatures. (for example, the last edition of Codex: Space Marines had Storm Talons added when that model was released)
I hope that helps.
- Eddie
So there you go. The codex is essentially a direct copy to prevent confusion because of things being drastically different between books. Also we have a small clue there that we may see updates when books get updated and options change.