Because there are 2 different species glued together. What you point to is a 1/2"X1/2" piece of black walnut glued to a 1/2"X1/2" piece of hard maple. The other specie represented is domestic cherry, that would be the tan parts. There are 12 fabricated units. Each unit is made from 4 individual pieces of material. Each species was milled to 1/2"X1/2". 

Some of my most creative moments have been when I've had to cover up an error in my woodworking.