Comment by Jtsummers

You elided the reason it's a valid set, "all different or all the same for each attribute separately"

The four attributes:

- Count: 3 in each; "all the same"

- Color: red, purple, green; "all different"

- Shape: diamonds, ovals, squiggles; "all different"

- Shading: empty, solid, striped; "all different"

So for each attribute the conditions are met, they are either all the same or all different.