If I'm reading your setup correctly, you have 5 accounts and A2/A3/A4 are all linked and A1 and A5 are not linked in any way.

The character levels on the accounts are:
A1: Level 70
A2: Level 20 ("toon 1")
A3: Level 20 ("toon 2")
A4: Level 13 ("toon 3")
A5: Level 20 ("toon 4")

Toon 1 did not receive triple exp, because he's linked to toon 3, who is lower level. You only receive triple exp if you are equal or lower than the linked character.
Toon 2 did not receive triple exp, because you only receive the bonus when grouped with a linked account, not solo.
Toon 3 received full exp because it's linked and grouped with toon 1, and lower level. Not sure why he didn't get 300%.
Toon 4 did not receive triple exp, because he's not linked to anyone.

Having a level 70 in the group has no effect on quest exp, only kill exp.

Ensure in the worldofwarcraft.com account management that your accounts are, indeed, linked. You can test it out by making new level 1 toons on all 5 accounts, grouping, and killing something. Toons on accounts 2-3-4 should receive a "+x from Recruit a Friend" message.