Yup, way back in 1991, Zahn tried to explain clones in Star Wars, because no one seriously believed that Lucas would make more movies. Well, he did, so Zahn's clones looked wonky and out of place. And thus began The Great Retcon. The clones in Heir to the Empire were canonized as shittier clone experiments made by the Emperor that were clearly inferior to the clones that the Kaminoans had popped out. Indeed, the Zahn Clones ended up going totally crazy pants and were not exactly great Stormtrooper material.
This bootleg cloning technology, by the way, was kept on Wayland, in a mountain, in the Emperor's personal basement. Just a bunch of Spaarti Cylinders propped up in the corner by the Emperor's fishing poles and his Face Loran collector's plates.