The given best-game solution is not optimal: Black can delay mate for another move to give a forced win in 11 ply rather than in 9 ply. Optimal is: N4 n5 N6+ k7 R4 as given, but Black can delay mate with ... n3+, rather than ...k6. The remaining moves are then: K2 n5 N8 k8 R5#. Perhaps your minimax code doesn't include moves-to-mate in the move score?