public and private problem

 using UnityEngine;
 using System.Collections;
 
 namespace CompleteProject
 {
     public class EnemyAttack : MonoBehaviour
     {
         public float timeBetweenAttacks = 0.5f;     // The time in seconds between each attack.
         **public int attackDamage = 20;**               // The amount of health taken away per attack.
 
 
         Animator anim;                              // Reference to the animator component.
         GameObject player;                          // Reference to the player GameObject.
         PlayerHealth playerHealth;                  // Reference to the player's health.
         EnemyHealth enemyHealth;                    // Reference to this enemy's health.
         bool playerInRange;                         // Whether player is within the trigger collider and can be attacked.
         float timer;                                // Timer for counting up to the next attack.
 
 
         void Awake ()
         {
             // Setting up the references.
             player = GameObject.FindGameObjectWithTag ("Player");
             playerHealth = player.GetComponent <PlayerHealth> ();
             enemyHealth = GetComponent<EnemyHealth>();
             anim = GetComponent <Animator> ();
         }
 
 
         void OnTriggerEnter (Collider other)
         {
             // If the entering collider is the player...
             if(other.gameObject == player)
             {
                 // ... the player is in range.
                 playerInRange = true;
             }
         }
 
 
         void OnTriggerExit (Collider other)
         {
             // If the exiting collider is the player...
             if(other.gameObject == player)
             {
                 // ... the player is no longer in range.
                 playerInRange = false;
             }
         }
 
 
         void Update ()
         {
             // Add the time since Update was last called to the timer.
             timer += Time.deltaTime;
 
             // If the timer exceeds the time between attacks, the player is in range and this enemy is alive...
             if(timer >= timeBetweenAttacks && playerInRange && enemyHealth.currentHealth > 0)
             {
                 // ... attack.
                 Attack ();
             }
 
             // If the player has zero or less health...
             if(playerHealth.currentHealth <= 0)
             {
                 // ... tell the animator the player is dead.
                 anim.SetTrigger ("PlayerDead");
             }
         }
 
 
         public void Attack ()
         {
             // Reset the timer.
             timer = 0f;
 
             // If the player has health to lose...
             if(playerHealth.currentHealth > 0)
             {
                 // ... damage the player.
                 print("Attackdamage == " + attackDamage);
                 **playerHealth.TakeDamage (attackDamage);**
             }
         }
     }
 }

and the problem is at the [*] position

first if i use public ... the variable will not work in the next [*] one...

but if i change public attackdamage to private attackdamage and it works now

I want someone that can explain to me why this is happen and what cause it to be like this

thank you