The pits and valleys are spaced λ/4 apart. The reason there is such a stark difference in intensity is because this λ/4 spacing causes interference at pit->valley and valley->pit transition points. Of course this not a standard interferometer but rather a purpose build one. Wikipedia says:

> Interferometry is a technique which uses the interference of superimposed waves to extract information.

And a blu ray player directly uses interference of superimposed waves to extract information. It squarely fits in the homodyne category.