When does a Chain level up?

//ads -- adsterra.com -- native banner
?>
When killing the same kind of enemy you build up a chain and after a high enough chain it "levels up" where the drop icon changes and at high levels you can recover HP and gain status effects like Protect and Shell.
But when i was doing some grinding in the Estersands when i was killing Wolves i noticed that the chain did it's first level up at different numbers, once was on 22 and another was on 8.
so when does a Chain level up?
Pictures about "When does a Chain level up?"



Dazy Chain - Level Up (Official Audio)
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Mathias Reding, Archifashiondesign, Giallo, Artem Beliaikin