It’s an incredibly common question and point of confusion, so no worries. 
I suggested a change to how DoT effects were explained on cards around a year ago, but it went nowhere. I agree that it’s very confusing that they list absolute numbers but actually are clearly percentage based in practice.
This comes up often on the other end of the spectrum too, when people are confused why the DoT amounts on an unmaxed hero aren’t the full amounts, and also don’t increase when the Special Skill does — but do increase as the Attack stat increases while leveling the hero.
FYI, there’s also an unlisted 20% bonus to the Defense stat as well — both of those were determined based on observed damage amounts and reverse engineering of the Damage Calculation formula, as well as observations about DoT amounts. But HP is unaffected.
Linking this too for cross-reference: Have we ever figured out how stat boosts are calculated when defending?