I think there's a misunderstanding there. Mediumturquoise and turquoise are both located above cyan, which means they are brighter as the vertical axis is the lightness.

Also, to answer your question, colors are sorted by their hue, saturation and lightness values. So, it's actually a 3D color visualizer, where one dimension (hue) is reduced into just a slider at the top, and the other two dimensions are represented in a 2D grid.

Thanks for your comment!