User blog comment:B214/Volume 22 spoilers/@comment-30289101-20161022000600/@comment-30289101-20161022020402

I can accept that explanation as well. I'm not entirely sure if it fits in the context of the tournament though. Kuroka, who is a 2 Bishop demon (6 pawns) is being treated as a single bishop (3 Pawns) which indicates that the piece you register for the tournament isn't directly related to how strong you are. Bova only taking up 3 pawns for the sake of the tourny doesn't mean he'd only need 3 pawns to hypothetically become issei's real pawn, he could easily be truly worthy of a rook (or more) especially given his lineage.

The Impression I get is, for the sake of the tournament, pieces aren't representative of power (outside of distinguishing god class from not god class in the form of requiring multiple pieces) They're more like positions on a team. If the Rook is still considered a higher position even in that context, it seems odd that Kuroka wouldn't have been forced into that slot, given that her real worth is greater than a rook's real worth.