Well, that's not really how it works. What you're actually seeing is the effect of the way airfare is priced. In order to find a price for a given itinerary, you need to consider the cities and the flights. First you find a fare that's applicable to the cities you're interested in -- INN-SEA, for example. Then, you look for the class of service the custom has requested -- business class, in this case. You start with the cheapest fares and you work through them, looking for one that's applicable. Some fares are only good on the weekends, some fares are only good if you route through certain cities, etc.

Entirely separate from this, you're also grabbing the inventory for the flights that you're interested in: INN-FRA-SEA, for example. "Married segment" logic means that the inventory for INN-FRA-SEA is entirely independent of INN-FRA and FRA-SEA; there can be more discount seats, or there can be fewer. The final requirement for a fare to be applicable is that there has to be inventory available in the right fare buckets on the flights in question.

The rules that you're describing for P and Z fares could be driven by different fares being published or by different inventory. That's pretty standard and it's what I meant when I said (to another poster) that airfare has nothing to do with the costs of operating the flights. Instead, it's based on this giant collection of fares and the inventory for each flight that the airline has available.