I was under the impression that by having a smaller piston on the leading edge that it would help with pad taper wear.

http://www.trackpedia.com/wiki/Calipers
"Differential bore calipers -- As the surface of the rotor heats up, the clamping force of the pistons has to be increased to avoid brake fade. If the caliper has multiple pistons (or multiple pairs of pistons), the brake rotor surface is initially heated by the pistons pushing against the brake pad at the leading edge of the caliper, making the rotor surface hotter when it rotates back to the pistons closer to the trailing edge of the caliper. Therefore it helps if the pistons closer to the rear edge of the caliper are larger. Differential-bore calipers use smaller pistons up front, larger pistons toward the back."

If he used the L caliper on the right, he'd have the larger piston on the leading edge.