During the development of Windows XP, the visual design team were very cloak-and-dagger about what the final visual look was going to be. They had done a lot of research and put a lot of work into their designs and wanted to make sure that they made a big splash at the E3 conference when Luna was unveiled. Nobody outside the visual styles team, not even me, knew what Luna was going to look like.
On the other hand, the programmers who were setting up the infrastructure for visual styles needed to have something to test their code against. And something had to go out in the betas.
The visual styles team came up with two styles. In secret, they worked on Luna. In public, they worked on a "decoy" visual style called "Mallard". (For non-English speakers: A mallard is a type of duck commonly used as the model for decoys.) The ruse was so successful that people were busy copying the decoy and porting it to their own systems. (So much for copyright protection.)