Battle domain's feature. Decisive strike question.

Hi, I'm Solasta player from Korea.

While I play the game, I found some mechanism of this game that I can't understand.

Please give me some knowledge. Thank you.

Decisive strike, the feature says

"On a successful attack, Channel Divinity to add 1d6 to your damage and stun your target (save negates) Increases by 1d6 every 3 levels above 2."

So I thought next terms:

Lv 2 has +1d6

Lv 5 has +2d6

Lv 8 has +3d6 bonus damage

But my Lv 9 Cleric still has +2d6...

Is it a bug? or only my case?

What am I missing?