Those things are sometimes true, but in this particular case the full story is more complicated, and the lesson more interesting. From N. Marion Hage's interview with Barbara Remington:
I worked for Ballantine, and as a practice, always read the books before doing the artwork. I didn’t have this luxury with the Tolkien Books, something I wish I could have changed. Ballantine was in a hurry to get these books out right away. When they commissioned me to do the artwork, I didn’t have the chance to see either book, though I tried to get a copy through my friends. So I didn’t know what they were about. I tried finding people that had read them, but the books were not readily available in the states, and so I had sketchy information at best... If I’d had the time to actually read the books first, which was my habit to do, I’d have definitely drawn different pictures.
I'm detail-oriented to a fault, and it always drives me crazy when sci-fi tabletop RPGs, for example, have detailed text descriptions of some widgets flanked by illustrations of the same that look nothing alike. The thing is, it's not usually the artist's fault, but a casualty of the way the industry operates.
Say the Shadowrun line editor decides he wants a book about robots to go to press in six months. He commissions a writer and some artists and gives each of them a bare outline and a deadline. He gets the commissioned material, lays it out, sends it to the printer, bam. If the artists don't start 'til the writer's finished, they miss their deadline, the book isn't available in time for Christmas/GenCon/whatever, it fails to turn a profit, there's no money to fund the next year's books, and the company goes under.
Obviously I'm simplifying and exaggerating a bit, but niche publishers like that really do operate hand-to-mouth a lot of the time. It's not quite as bad at the larger outfits, but margins in the fiction industry are always razor-thin, and cover artists are quite often simply not given enough time to read a book and still complete the cover before the deadline, so they do the best they can.