Why arrays don't set on void Start() ?

So I have problem, my arrays don't set on spot.

 public static int currentHealth;
     public static int currentXP;
     public static int currentLevel;
     public static int currentAttack;
     public static int currentDefense;
     public static int maxHealth;
     public static int currentMana;
     public static int maxMana;
     public static int[] needXPForLevel;
     public static int[] defenseOnLevel;
     public static int[] hpOnLevel;
     public static int[] attackOnLevel;
     public static int[] manaOnLevel;
     public static bool set;
 
 
     //Don't destroy:
     private static bool created = false;
     void Start () {
         if (!created) {
             GameObject.DontDestroyOnLoad (transform.gameObject);
             created = true;
         } else {
             Destroy (gameObject);
         }
     }

I have this class with variables and when i'm setting them from another class, they don't set on spot.

 void Start () {
         Save.needXPForLevel = needXPForLevel;
         Save.defenseOnLevel = defenseOnLevel;
         Save.hpOnLevel = hpOnLevel;
         Save.attackOnLevel = attackOnLevel;
         Save.manaOnLevel = manaOnLevel;
     }

When i try to use those arrays I get error that they're out of index.

 expText.text = "" + Mathf.Floor (((float)Save.currentXP / Save.needXPForLevel [Save.currentLevel + 1]) * 100) + "%";

Well, i tried to fix that, and it fixed it:

 if (Save.needXPForLevel.Length == 0) {
             Save.needXPForLevel = needXPForLevel;
             Save.defenseOnLevel = defenseOnLevel;
             Save.hpOnLevel = hpOnLevel;
             Save.attackOnLevel = attackOnLevel;
             Save.manaOnLevel = manaOnLevel;
 }
 expText.text = "" + Mathf.Floor (((float)Save.currentXP / Save.needXPForLevel [Save.currentLevel + 1]) * 100) + "%"; //No error this time

So i have question, why my arrays doesn't set on start ?