It's skill point.

When u are on free ongs then delevel about from 20 to 10 and keep ur mastery 0.
When u have some friend to farm u every day(not a random turk) then level to 26 or 28 and delevel to 18 or 20.
U could keep ur mastery 0 in this case too but u may also keep it 9levels below your lowest level, so after deleveling ur gap is still 9.