That is the way. If you want cabin E and it is available for both short legs, a PVP or skilled T/A may be able to get it released it for the dual itinerary. When they are dual-porting, it would make it easier for Princess if it was all guarantees, but they have kept specific cabins available.
The other alternative is to have a loose booking arrangement, and the risk of passengers denied boarding due to overbooking.
The guarantee categories are usually the cheaper ones for each cabin type. Seems that passengers not only want the cheapest fare, but also their choice of cabin. That isn't always possible. In most instances, specific cabins are available for most cabin types on Princess, but once they have sold, they become guarantee only to allow them the flexibility to fill all available cabins.