Defence in Maple is called PDR. I don't remember what it really stands for, but it's something like percentage damage reduction or percentage defence rate or physical defence reduction, etc.
The point is that it's not a value, it's a percentage. A rate. When people talk about ignoring defence, what really happens is that the item/skill ignores a percentage of the mobs defence, which itself is a system which reduces your damage by a certain percentage. To ignore a set % of the mob's defence is to ignore a % of a %.