Number of Vector3 cords in list should be changed, but list is not changing?

Hello, I'm having an issue where I am creating 10,000 Vector3 cords, and then checking them off as objects are instantiated in those positions, but they are not being removed from the list even though the code is there.

     public GameObject[] PlantObjects;
     public GameObject[] GrassObjects;
 
     public GameObject Created;
 
     public List<Vector3> UsedPositions = new List<Vector3> ();
     public List<Vector3> UnusedPositions = new List<Vector3> ();
 
     public int[] SolidNumberofObjectsToGenerate;
     public int[] PlantNumberofObjectsToGenerate;
     public int[] GrassNumberofObjectsToGenerate;
 
     public int MapWidth;
     public int MapLength;
 
     private float Cord_X = 0;
     private float Cord_Z = 0;
 
     void Update () {
         for (;MapWidth * MapWidth > UnusedPositions.Count;) {
             UnusedPositions.Add (new Vector3 (Cord_X, 0.5f, Cord_Z));
             Cord_X += 1;
             if (Cord_X >= MapLength) {
                 Cord_X = 0;
                 Cord_Z += 1;
                 if (Cord_Z > MapLength) {
                     Cord_Z = 0;
                 }
             }
         }
         if (UnusedPositions.Count == MapWidth * MapLength) {
             Cord_X = 0;
             Cord_Z = 0;
         }
 
         for (int i = 0; i < SolidNumberofObjectsToGenerate.Length; i++) {
             int NumberOfInstances = SolidNumberofObjectsToGenerate [i];
             for (int j = 0; j < NumberOfInstances; j++) {
                 if (SolidNumberofObjectsToGenerate [i] > 0 && Cord_Z < MapLength) {
                     Created = Instantiate (SolidObjects [i], new Vector3 (Cord_X, 0.5f, Cord_Z), Quaternion.identity) as GameObject;
                     UsedPositions.Add (Created.transform.position);
                     UnusedPositions.Remove (Created.transform.position);
                     SolidNumberofObjectsToGenerate [i] += -1;
                     Cord_X += 1;
                     if (Cord_X >= MapLength) {
                         Cord_X = 0;
                         Cord_Z += 1;
                         if (Cord_Z >= MapLength) {
                         }
                     }
                 }
             }
         }
 
         for (int i = 0; i < PlantNumberofObjectsToGenerate.Length; i++) {
             int NumberOfInstances = PlantNumberofObjectsToGenerate [i];
             for (int j = 0; j < NumberOfInstances; j++) {
                 if (PlantNumberofObjectsToGenerate [i] > 0 && Cord_Z < MapLength) {
                     Created = Instantiate (PlantObjects [i], new Vector3 (Cord_X, 0.5f, Cord_Z), Quaternion.identity) as GameObject;
                     UsedPositions.Add(Created.transform.position);
                     UnusedPositions.Remove (Created.transform.position);
                     PlantNumberofObjectsToGenerate [i] += -1;
                     Cord_X += 1;
                     if (Cord_X >= MapLength) {
                         Cord_X = 0;
                         Cord_Z += 1;
                         if (Cord_Z >= MapLength) {
                         }
                     }
                 }
             }
         }

         for (int i = 0; i < GrassNumberofObjectsToGenerate.Length; i++) {
             int NumberOfInstances = GrassNumberofObjectsToGenerate [i];
             for (int j = 0; j < NumberOfInstances; j++) {
                 if (GrassNumberofObjectsToGenerate [i] > 0 && Cord_Z < MapLength) {
                     Created = Instantiate (GrassObjects [i], new Vector3 (Cord_X, 0.5f, Cord_Z), Quaternion.identity) as GameObject;
                     UsedPositions.Add(Created.transform.position);
                     UnusedPositions.Remove (Created.transform.position);
                     GrassNumberofObjectsToGenerate [i] += -1;
                     Cord_X += 1;
                     if (Cord_X >= MapLength) {
                         Cord_X = 0;
                         Cord_Z += 1;
                         if (Cord_Z >= MapLength) {
                         }
                     }
                 }
             }
         }
         Debug.Log (UnusedPositions.Count);
     }
 }

As you can see, I am debugging the number of UnusedPositions that are left after all the code is run, but it always returns 10,000, even when I generate 10,000 objects into all those positions. I don't understand why the positions are not being removed from the list. Thanks,