Touchdown/hydroplane and anti-skid modes provide the same function - preventing wheel lockup, however the touchdown and hydroplane modes are focused on assuring wheel spin up by preventing early heavy braking, while anti-skid logic keeps the tires at their best slip ratio for max traction throughout the remainder of the rollout.
The split logic modes provide a bridge between high speed, low traction anti-skid right after touchdown and the rest of the rollout braking control after wheel spin up has occurred by assuring that at least one pair of wheels (1/3 - LO/RI or 2/4 LI/RO) will find traction.
The Inner/Outer pairing enhances wheel speed sensor correlation, avoiding the situation where two tires are slipping at the same rate, but below the GS difference trigger, therefore sending data to the comparator that would otherwise command continued pressure application, when in fact, modulation is required.