Yep, it's just the way the math works out.
With reflective phase grate diffusors, there are a variety of well depth sequences you can use. Some work better than others at different frequencies and angles of incidence. But to work right they have to be built precisely. If something is off then you will lose some diffusion functionality.
Put it this way: nothing on the Gotham is there purely accidentally. Lots of prototyping and R&D went in to their design.
For more detail on this stuff, see the "Decoding Diffusion" article linked above, as well as:http://www.gikacoustics.com/diffusion-dimension-download-1d-2d-scattering/