No you don't understand the concept. It isn't damage amount. It is the percent of the damage dealt that will be that type of damage. If you put a 20 in the fire box, 20% of all the damage the mobile deals will be fire damage. So if the mobile deals 100 damage, 20% of that will be fire and the other 80% will be whatever you put the other 80% in.
So the percent is out of its total damage. So you can't have 110% of the monster's total damage being fire damage, since that isn't possible. It is a hard concept to explain and I know I am not doing a good job of it. But trust me, the damage types cannot add up to over 100% or under 100% because it may error or have some unknown effect.