To explain:
If you level 1 character up to 60, he grants 30 levels to AccountC.
AccountC now has 30 more levels, so he can grant 15 levels to AccountB.
AccountB now has 15 more levels, so he can grant 7 levels to AccountA.

Now if you level all characters up to 60:
AccountD grants 30 levels to AccountC. AccountC now has 60 +30 levels.
AccountC grants 45 levels to AccountB. AccountB now has 60 + 30 +15 levels.
AccountB grants 52 levels to AccountA. AccountA now has 60 + 30 + 15 + 7 levels.
AccountA grants 55 levels to the Veteran.

Or something like that.