Unity: Sprite stretch issue

I have a simple png square 8px x 8px. In Unity I’ve left the pixels per unit at 100 and added the image to the scene to create a prefab of the item.
I then add four items to the screen like so:

    var block = Resources.Load("Prefabs/Block", typeof(GameObject));

    Instantiate(block, new Vector3(1f, 0f), Quaternion.identity);
    Instantiate(block, new Vector3(0f, 1f), Quaternion.identity);
    Instantiate(block, new Vector3(2f, 0f), Quaternion.identity);
    Instantiate(block, new Vector3(0f, 2f), Quaternion.identity);

The results (as shown in the image below) seem to stretch the image dependant on where the prefab is placed. Is this normal behaviour? Is there a way round this or something I’m missing?