Since we're dividing genAddedHealth at this point by 20, and genAddedHealth is effectively healthMultiplier * difficulty, we can think of this as "Add healthMultiplier HP to the mob's total health every 20 levels."

This doesn't account for the mob's base HP at all, and doesn't scale against the mob's health. It only adds to it.

Instead, the mob's health should be set to their baseMaxHealth + baseMaxHealth * genAddedHealth, where genAddedHealth is healthMultiplier * difficulty.

e.g.
30 HP with .5 multiplier at 20 difficulty becomes
30 + 30 * .5 * 20 => 330 HP

20 HP with .1 multiplier at 20 difficulty becomes
20 + 20 * .1 * 20 => 60

Note, this is for 1.12