speed diffrence

This code is connected to for game-objects but for some reason all of the game-objects get different speeds and they all started with the same speed.(the tags are set correctly)

using UnityEngine; using System.Collections;

public class Arrow : MonoBehaviour {

 public GameObject target;
 public GameObject target1;
 public GameObject target2;
 public GameObject target3;
 public GameObject target4;
 public GameObject target5;
 public GameObject target6;
 public GameObject target7;



 private int speed = 5;

 private int ranN;



 void Update () {
     transform.position += Vector3.back * speed * Time.deltaTime;
 
 }







 void OnTriggerEnter (Collider other) {
 
     if (other.gameObject.CompareTag ("reset")) {
     
         ranN = Random.Range (1, 8);
     
     }
     if (ranN == 1) {
         transform.position = target.transform.position;
         speed = speed + 1;
     } else {
         if (ranN == 2) {
             transform.position = target1.transform.position;
             speed = speed + 1;
         } else {
             if (ranN == 3) {
                 transform.position = target2.transform.position;
                 speed = speed + 1;
             } else {
                 if (ranN == 4) {
                     transform.position = target3.transform.position;
                     speed = speed + 1;
                 } else {
                     if (ranN == 5) {
                         transform.position = target4.transform.position;
                         speed = speed + 1;
                     } else {
                         if (ranN == 6) {
                             transform.position = target5.transform.position;
                             speed = speed + 1;
                         } else {
                             if (ranN == 7) {
                                 transform.position = target6.transform.position;
                                 speed = speed + 1;
                             }
                         }
                     }
                 }
             }
         }
     }
     

 }
 }