Testing JSON with SQL Server 2016 and I'm confused on the seemingly extraneous square bracket around the child table

JsonPackUnpack.sql

SET NOCOUNT ON;
IF OBJECT_ID('tempdb..#Sales') IS NULL
BEGIN
    CREATE TABLE #Sales
    (
        RecordKey int NOT NULL
    ,   SalesDate date NOT NULL
    ,   CONSTRAINT PK_tmp_Sales PRIMARY KEY (RecordKey)
    )
    INSERT INTO
        #Sales
    SELECT
        1 AS RecordKey
    ,   CAST('2017-01-13' AS date) AS SalesDate;
END

IF OBJECT_ID('tempdb..#SalesDetail') IS NULL
BEGIN
    CREATE TABLE #SalesDetail
    (
        RecordKey int NOT NULL
    ,   SalesRecordKey int NOT NULL
    ,   Quantity int NOT NULL
    ,   CONSTRAINT PK_tmp_SalesDetail PRIMARY KEY (RecordKey)
    )
    INSERT INTO
        #SalesDetail
    SELECT
        1 AS RecordKey
    ,   1 AS SalesRecordKey
    ,   100 AS Quantity;
END

SELECT
*
FROM
    #Sales AS S
    INNER JOIN
        #SalesDetail AS SD
    ON SD.SalesRecordKey = S.RecordKey
FOR JSON AUTO;


DECLARE
    @json nvarchar(max);

/*
SELECT @json = N'[
    {
        "RecordKey":1,
        "SalesDate":"2017-01-13",
        "SD":[
            {
                "RecordKey":1,
                "SalesRecordKey":1,
                "Quantity":100
            }
        ]
    }
]';
*/

SELECT 
    @json = 
(
    SELECT
    *
    FROM
        #Sales AS S
        INNER JOIN
            #SalesDetail AS SD
        ON SD.SalesRecordKey = S.RecordKey
    FOR JSON AUTO
);


SELECT 
*
FROM
    OPENJSON(@json) 
    WITH
    (
        RecordKey int '$.RecordKey'
    ,   SalesDate date '$.SalesDate'
    -- Why does this not find it?
    -- Because the AUTO defines SD with square brackets
    -- "SD":[{"RecordKey":1,"SalesRecordKey":1,"Quantity":100}]
    ,  Quantity int '$.SD.Quantity'
    )AS OJ;


SELECT @json = N'[
    {
        "RecordKey":1,
        "SalesDate":"2017-01-13",
        "SD":
        {
            "RecordKey":1,
            "SalesRecordKey":1,
            "Quantity":100
        }
    }
]';

-- Convert JSON Data to Rows and Columns with OPENJSON
-- https://msdn.microsoft.com/en-us/library/dn921879.aspx
SELECT 
*
FROM
    OPENJSON(@json) 
    WITH
    (
        RecordKey int '$.RecordKey'
    ,   SalesDate date '$.SalesDate'
    -- Now it works
    ,   Quantity int '$.SD.Quantity'
    )AS OJ;