I think your source of confusion is the relationship between fares and booking classes. Just because a route has a certain booking class (in this case, Z) it doesn't necessarily mean that there is a corresponding fare that uses that class.

Although there are some classes like Z in this case that has no corresponding fare solely for that segment, there are often through-fares to other destinations and other purposes which require that class.

In this case, in all likelihood there is no Z fare specifically between LCY and LCA.