If you make several calls to the Image Editing API with the same image, you will notice that it will generate a different uncropped subject every time.
In order to reduce the randomness, you can set a fixed value for the seed that the API's random generator will use. The value of the argument uncrop.seed can be any positive integer.
If you provide a value for uncrop.seed, then the API should produce similar-looking subject uncroppings for the same image.