Why is there limited damage for Volibear W

So, I just played Volibear in Aram and it showed that he does 15% of his hp as damage when he bites. However, that wasn't the case, because it showed that I only do 400 damage (max hp damage from the 15%) but it should be 700~ because I had 4700~ HP.

Hope it is a script error, not a limit to Volibear power.