We were at dinner with the developer of the Biomat and asked the same question. What he told us was with the smaller surface area, the tourmaline rows gave a better heat profile. The larger mats did not need that. They experimented using a blind test, where people sat on mats with different configurations. They compared configurations with all amethyst, alternating rows, and doubling rows. Ultimately, they found the alternating amethyst and tourmaline was perceived to provide the best feeling.
All Biomats today also have a layer of tourmaline powder mixed with other elements to create negative ions. You do not see that from the outside.