lcc is not a Constant property of the clCore class, so each instance of clCode has its own lcc property.
So which clCode instance's lcc property do you want to use in the get.dpu method in the clPrimary class?
As an analogy, it doesn't really make sense to ask for human's name. It makes sense to ask for one particular human's name.
Now if one of those properties of the clPrimary class instance whose dpu property you're asking to compute contains a clCore class instance, asking for the lcc property of that instance makes sense.
In the human analogy, while it doesn't make sense for you to ask for human's mother's name it would make sense for you to ask for my mother's name. In this analogy, I am an instance of the human class.