If you compare the sketch below with the final painting (Seton's Scout - Torment uncommon - by Mark Romanoski), you'll see one major component missing: the saddle.
When the art descriptions were being set out for Odyssey and Torment, no one thought it was necessary to tell the artists that Centaurs would not wear saddles. As sentient beings that do not let people ride them, it makes some sense that they wouldn't have saddles. But it must make a certain amount of sense for them to have saddles as well, considering every single artist painted them with saddles.
The saddles were removed via the magic of Photoshop before the cards were printed.