Some more information about "indexes" added to parameters when incoming data contains array of hashes

I am using Ruby on Rails 4.1 and I would like to know some more about "indexes" added to parameters when incoming data contains array of hashes.

That is, for instance, when I run the following code (as an example, the code executes a simple AJAX request to my application)

var my_data = [
  { "a1": 1, "b1": 2 },
  { "a2": 1, "b2": 2 },
  { "a3": 1, "b3": 2 },
];

$.ajax({
  type: "POST",
  url:  "http://0.0.0.0:3000/path.json",
  data: { "my_data": my_data }
});

Then Rails parses the following parameters by "automagically" adding "0", "1", "2" indexes:

{"my_data"=>{"0"=>{"a1"=>"1", "b1"=>"2"}, {"1"=>{"a2"=>"1", "b2"=>"2"}, {"2"=>{"a3"=>"1", "b3"=>"2"}}

1. Why Rails adds those indexes?
2. When Rails is supposed to add indexes? And when not?
3. How to avoid Rails to add indexes?