Why is a curly bracket required before the "$divideā€ in the following? Is it because of the square bracket? Is it because of a mongodb bug?

red_green_blue_divide_by_100_and_pow = { 
    '$project' =>
    {
        "red_pow" => { "$pow" =>
        [{"$divide" => ["$top_colour.red", 255.0]}, 2.2]},
        "green_pow" => { "$pow" => [
            {"$divide" => ["$top_colour.green", 255.0]}, 2.2]},
        "blue_pow" => { "$pow" => [
            {"$divide" => ["$top_colour.blue", 255.0]}, 2.2]}
      }
    }

But not before the "$pow" in the following:

red_green_blue_float_avg = { 
    '$project' =>
  {
        "_id" => 0,
    "red_float_avg" => {
          "$multiply" =>
          [255.0, "$pow" => ["$red_linear_avg", 1.0/2.2]]},
    "green_float_avg" => {
      "$multiply" =>
          [255.0, "$pow" => ["$green_linear_avg", 1.0/2.2]]},
    "blue_float_avg" => {
          "$multiply" =>
          [255.0, "$pow" => ["$blue_linear_avg", 1.0/2.2]]}
  }
    }