Thanks to Coop from the World of Warships Global Fanbase Facebook group for sending me this. His words:
So, apparently, when the fixed the match maker, they forgot to tell the MM to treat the Mikasa as a battleship:
EDIT: It was changed by WG cause Mikasa is the only tier II BB:
„Exception: battleship Mikasa . Being the only Tier 2 battleship of the game , it is weighted with Tier II cruisers. While this does not affect the balance , it reduces significantly the waiting time in the queue for Mikasa.”