I think you are confusing the word “stores” with “holds”. A wire holds a value which is driven on it. For example if you say assign wireA= regA + regB, the wireA always hold a value which is the sum of values “stored” in regA and regB.
If a wire is not driven at any point in the simulation, it holds a default value of “z”. (Remember this is a 4-state wire and therefore it has to hold one of the 4 values).
Hope this helps
Logie Ramachandran
Accelver Systems Inc