Forgive me for such a dumb question, but the lines look as if the material were carved away from them - that they're not just grooves in the surface.
As if the grooves were carved to delineate the design, and then material surrounding them ground away.
That would take a hell of a lot of work and patience, would it not?
Or maybe it's just the shadows in the photo misleading me...[LATER: Yes.]

Plus, it's obviously a tiger, and not some vague arty rendition of a Shang beast or dragon, which makes it more "real" - the artist actually knew what a tiger looks like?