It looks like there are a system to the piece count. USA, UK and ANZAC got exactely the same number of pieces, and Japan got a dobble set, except on Tac’s and Mech’s. Its like when Italy got only 2 cruisers in AA50 ed. The rare pieces are rare, so because of that we need to buy 2 or more copies.
Here is the piece count:
USA/UK/ANZAC:
Inf 20, Mech 6, Art 6 and Tank 8 (USA got only 6 Tanks)
Ftr 8, Tac 6 and Bomb 6
Tranny 6, Sub 6, DD 6, CA 4, BB 4 and CV 2 (USA got 4 CV’s)
Japan got dobble of that, like 40 inf, 12 Art, 12 Tanks but only 6 new Mech’s.
Japan got 16 Ftr but only 6 of the new Tac’s
Japan got dobble of all ships, since there are no new ship pieces.