This is an obsevation not a critism
Its something I've never noticed before but last week I got the choas codex and Horus heresy betrayal. I read the choas codex with great gusto and was impressed. While the book wasn't worth the $83 price tag, it is a great book, the art work is good the army list seems fun and well thought out and none of the fluff made me sudder with embarrassment. Phil Kelly even made abaddon not sound like a loser. Great book it has me buying miniatures that I wasn't planning to get. Then I picked up the tome which forge world had sent me. About 290 pages, I read it in one day. ALL OF IT.
It's funny, I see people talking about who the better codex writer is Phil Kelly or Matt ward, the church of Kelly exists and for good reason, the Matt ward defense league are out there as well. But the simple fact is that neither can really hold a candle to Alan Bligh. Anyone who had read the badab war books knows what I'm talking about. The fluff, the units and characters and the army lists are all so well done.
So what do I mean by my title? Well it comes down to whom the book has been marketed. GW targets the teen market, it's no serect and so the fluff in the codexs are written in a style that takes that into mind. The language used and still appeals to teens, now this is fine and I quite enjoy it. It's like reading a young adult book. I loved the rework of the necron fluff and now how Kelly has reworked choas so they are no longer the diet coke of evil, the best example of this writing it dragio's entry.
All my friends (not the largest sample I will admit) having read his fluff disliked him. And asked "why couldn't we have just a normal grand master?" I myself nevered used dragio as himself and wrote up fluff for a grand master who then ran around with dragio's stats and gear. I am yet to meet a boy between the ages of 12 to 18 who doesn't think dragio's story is the coolest ever....
The forge world books on the other hand read as military histories (the exception being volume 8 which is the worst written one). Again the style and language change. And the themes are much more quiet and restrained. The character are real and the whole book is much more focused.
It is worth noting that part of dragio's fluff problem is that it is poorly written and so you could blame at Matt ward's door. But it doesn't change the style of the writing that is used. The black Templar codex was written by graham McNeil and it does not read anything like his novels.
Also a note on price of Horus heresy betrayal, it is a A4 leather bound hardback book with silver guilded pages. If the book was a real military history in this format you would be paying £300 for it and it wouldnt have any gaming rules in it. Trust me I know.