Right. This also biases the test toward certain kinds of algorithm. The implicit requirement that the thing being learned should be something you can see (and c

Right. This also biases the test toward certain kinds of algorithm. The implicit requirement that the thing being learned should be something you can see (and check) by looking at pictures like these pretty much guarantees that simple "proximity-based" approaches will do well. So it's no surprise that we see nearest-neighbours and radial basis functions looking good in these tests.