Depending on how many games are included, then the top hammering could actually account for the whole decrease.
If there's 5 games - and I don't know what the sample is - then going from 142-0 to 63-0 in one game reduce the 2003 average from -45 to -29. Then in 2007, reducing one game from 108-13 to 63-0 reduces the average defeat margin from -39 to -30. In 2011, 87-0 to 63-0 over 5 games reduces the margin from -38 to -33.
So in that case - and again, there's probably more than 5 games involved - it'd just be one hammering reducing, and everything else staying the same. Even if there's 15 games, it would account for a good chunk of the reduction.
Is there a Tier 3 at all? Or is Tier 2 everyone from Japan to Russia?