He's trying to copy the original rule, where it was an extra d6 for every 10 feet cumulative with the steps before it, so:
10ft fall = 1d6
20ft fall = 3d6 (2d6+step 1)
30ft fall = 6d6 (3d6 + step 2 + step 1)
etc.
At least how Frank Mentzer explained the early gaming sessions, even if I can't...