Max sail size for a board

[viking]

Hi

There is something I do not understand clearly. How the sail range announced for one board (particularly the max one) is determined?
Because in my opinion, there are some discrepancies from one board to another.

For example, the 68cm width boards: the carve 122 is given with 8.0 max as the Aero 117 which has all its dimensions below the carve ones is given for 8.5, why? On the same time, the Isonic 111, with less volume, is given up to 9.0...