Rewarded video ads rewarding player too much

     using UnityEngine;
     using System;
     using GoogleMobileAds.Api;
     using UnityEngine.UI;

     public class ReklamRewardedVideo1 : MonoBehaviour
    {
     public Button reklamButton;
     private int gemsReklam;
    
     
     void Start()
     {
        
         MobileAds.Initialize("ca-app***************************************");
         YeniReklamAl(null, null);
 
         RewardBasedVideoAd reklamObjesi = RewardBasedVideoAd.Instance;
         reklamObjesi.OnAdClosed -= YeniReklamAl;
         reklamObjesi.OnAdClosed += YeniReklamAl; // Kullanıcı reklamı kapattıktan sonra çağrılır
         reklamObjesi.OnAdRewarded -= OyuncuyuOdullendir;
         reklamObjesi.OnAdRewarded += OyuncuyuOdullendir; // Kullanıcı reklamı tamamen izledikten sonra çağrılır
     }
     public void reklamReward()
     {
         reklamButton.enabled = RewardBasedVideoAd.Instance.IsLoaded();
         RewardBasedVideoAd.Instance.Show();
         reklamButton.enabled = true;
         
     }
     
     
 
     public void YeniReklamAl(object sender, EventArgs args)
     {
         RewardBasedVideoAd reklamObjesi = RewardBasedVideoAd.Instance;
 
         AdRequest reklamIstegi = new AdRequest.Builder().Build();
         reklamObjesi.LoadAd(reklamIstegi, "ca-app**********************************");
     }
 
     private void OyuncuyuOdullendir(object sender, Reward args)
     {
         gemsReklam = PlayerPrefs.GetInt("gems", 0);
        PlayerPrefs.SetInt("gems",gemsReklam+25);
     }
 }

here is my rewarded video ads code. It's been called when player clicks a button. It's working properly. But my biggest problem is that: It never gives 25 gems as a reward. I want it to give 25 gems as reward. but by using this code , I get 50 gems as a reward at first rewardedads, 50 gems at second click. After that I go to a level and play game a little bit. I came back to this scene and watch 3rd rewardedads and it gives 125 gems, 4th reward is 175 gems.. It goes in that way. never gives 25 gems as reward and it is like giving rewards randomly. please help me to fix this. what am I missing here?