The parent company has a factory for private label and diffusion lines, and another factory for Isaia.
But I'm no expert—that's just what the creative director of Eidos posted once after he kept seeing people say "made by Isaia"—so I'll bow out here.
Made by the factory that doesn't make Isaia, but is owned by the parent company. And not to the same standards. Also a factory and a brand are such different things anyway, right? But I promise to shut up about it from here on out :)