I thought it would be something like that, Like Bering Sea, SoOchotsk cut out.
Haven't read this thread much or forgotten about the content.
Looking back, I saw your Feb map on the Cryosat domain. It cuts out a lot of other peripheral seas: the CAA, Hudson, EGS, Barentsz and Kara!
Leaving only 7.289 Mkm2. according to my CAD 'Total Arctic Ocean' map.
Not hard to guess how it arrives on 15000km3. And I remember, a lot of the PIOMAS volume at maximum came from extent growth in the Baffin/Barentsz/Kara seas.
The wait will be for Cryosat to assess the effects of this summer on coming winter's volume...