Is this particular example proofgrade?
With Baltic birch ply for example you sometimes engrave away the top layer and expose the middle layer. The middle layer is wood in Baltic and will sometimes have knots and other grain structures that are revealed by engraving.
Example:
You can see variation in the middle layer in the ocean areas where I engraved the entire top layer away.
These variations are less common in proofgrade or other mdf-core plywoods.