Its kinda hard to fully explain, but its all probabilities, white skills tend to benefit for that specific role, because for example you dont expect your striker to tackle opponents attacker. - Therefore, no benefit to put any points on tackle.

However if probability made your striker tackle an opponent, and when your tackle is low (probability to commit a foul would be much higher compare to a DC that has high tackling.

Though mostly training white skills cant go wrong and are the most optimum if you ain't sure what you doing.