Has anybody ever noticed that a lot of the Horus Heresy series' cover art is highly inconsistant with fluff?

In the attached thumbnail we can see the cover art for the upcoming Fallen Angels novel by Mike Lee. Now, Pre-heresy Dark Angels colours were black with a red winged sword. Why then, is their armour bone white with green?

Other such inconsistencies appear on Galaxy in Flames. All Marines on the front cover are Death Guard, yet they have two armour schemes. Yet never repaint their armour.

Another example is Horus Rising and False Gods. Both Luna Wolves, yet one scheme is silver and the other is white with black trim.

What is going on here? Surely the author should have to approve cover art and the artists should at least be briefed on the book they're drawing a principle feature for. It just seems terribly off, any thoughts?