Anyone's guess is as good as the next one's, but yeah, that is my take on it. They are re-using old ideas from A330 fleet, to have different capacity cabins. In theory this is good so they can adapt availability to demand. I reality it does not work on such a small fleet.
They swap aircraft almost every day due to tech or late incoming. Swap a J/46 A350 for a J/32 A350 and they are facing a situation where they might have to bump 14 business class passengers. First on the bump list is upgraders, awards and corporate fare fliers, so you can be sure it is hitting the loyal Finnair customers the most.
As for the seasonality in demand, they talked a lot about the flexible config of the A350 cabins where they can change cabin config at will. Has anyone ever heard of them suing that feature?