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I don't know if this is how it still works, but I remember many years ago reading an article in WD about how they produce the art.

Basically they get 3 or 4 artists and tell them "we want an image of a thousand badass marines advancing towards the viewer, with one extra badass marine in the middle", and sends them all away.
A little while later the artists submit their work and GW picks the one they like the most.
Accuracy be damned, thats why John Blanche managed to get away with submitting nothing but doodles for years.

Its all done freelance out of house. I doubt many of the freelance artists have actually read the rules.
If GW wants an image of say, a Dreadnought, they'll provide the artist with shots of a couple of models/parts and leave them to it.
The guy drawing/painting the picture has no idea that a certain weapons combo is illegal in the codex.
GW don't care as long as the image looks cool enough to help shift units.

Its all just flavour, they've ret-conned so much of their stuff I don't think they know what they've changed, and what they haven't anyway.
Re-using old art is cheaper than commisioning enough new stuff to fill a codex, so just slip in a picture or two from RT/2nd/3rd etc, the kids won't know any better.