The intention is that colors of the same number have a similar luminance, so you can easily adjust contrast and hue independently while designing. These schemes

The intention is that colors of the same number have a similar luminance, so you can easily adjust contrast and hue independently while designing. These schemes also keep a relatively consistent saturation profile across the palette. It's the kind of thing professional designers do intentionally, but set up in a system that makes it easier to get a "designed" look without much effort.