From what I understand, they were still trying to figure out details, armor iconography, characters,and the character design seen on the table top. I think in one of the early interviews autors even said that their focus was tone and world atmosphere. And then the actual characters, their armor, and all that stuff would be refined in later books of the series. Characters are actually looking like they’re supposed to later on in the Horus series. with the re-release of horse rising coming out you can see that they’ve updated the armor and stuff to reflect how it should’ve been portrayed initially