Why does our AI play worse at even levels of depth? We are building an AI to play a board game. Leaving aside the implementation, we noticed that it plays worse when we set an even (2,4,6,...) level of depth. We use a minimax depth-first strategy.
Do you have any ideas why it behaves like that?

Edit: for example if we set a game between an AI with 5 levels of depth and an AI with 6 levels of depth, the first one usually wins (and this is weird).
When the number of levels is odd, it means the first player can do one more extra movement on the board. As it is an extensive form game, when decide using backward induction, as the last and the first move are for the first player, so the first player can act better than the situation that the second player will make the last move.