Why is my for loop not instantiating object at correct possision from List

My code complies and runs however It is not working as intended. The desired outcome is for a "testTile " to be placed for every Vector3 in gridPosition. This is the result of the current code.

http://imgur.com/2BgvTLY

As you can see gridPositions contains 6 different Vector3's and 6 tiles where instantiated. There is no tile at (2,1,0) and instead an extra tile at one of the other positions. I suspect that the problem lies in the for loop but I'm out of ideas for what to try.

I have tried to add +1 or -1 to various parts of the for loop but I get an extra undesired tile and an "ArgumentOutOfRangeException" message. I thought it would be solved by adding a nullable to the end of the list but that can't be done (so far as I know). Just what is going on here, and is there a way to right this where I get all 6 tiles to match up with the correct vector3?

  public class TestLevelManager : MonoBehaviour
     {
         public class Grid
         { 
             public int sizeX;
             public int sizeY;
             public List<Vector3> gridLocations;
           
             public Grid(int x, int y, List<Vector3> list)
             {
                 sizeX = x;
                 sizeY = y;
                 gridLocations = list;
             }
     
             public void CreateGrid()
             {
                 for (int x = 0; x < sizeX; x++)
                 {
                     for (int y = 0; y < sizeY; y++)
                     {
                         Vector3 _position = new Vector3(x, y, 0f);
                         gridLocations.Add(_position);
                     }
                 }
             }
         }
     
         void SpawnTiles()
         {
             for (int i = 0; i < gridLocations.Count; i++)
             {
                 Instantiate(testTile[0]);
                 testTile[0].transform.position = (Vector3)gridLocations[(i)];
             }
         }
     
     
         private int rows, colums;
         public List<Vector3> gridLocations;
         public GameObject[] testTile;
     
     
         void Awake()
         {
             List<Vector3> gridLocations = new List<Vector3>();
         }
     
         void Start()
         {
             rows = Random.Range(1, 4);
             colums = Random.Range(1, 4);
             Grid cellGrid = new Grid(rows, colums, gridLocations);
             cellGrid.CreateGrid();
             SpawnTiles();    
         }
     }