unset and json decode/encode

Question

In my project i have 2d array, stores it in the DB as a string and processed as required. after reading from the database looks like :

[[19,44,69,99],[26,27,39,49,70,80],[20,30,40],[47,57,67],[84,85,86,87]]

after json_decode() looks like:

Array
(
    [0] => Array
        (
            [0] => 19
            [1] => 44
            [2] => 69
            [3] => 99
        )

    [1] => Array
        (
            [0] => 26
            [1] => 27
            [2] => 39
            [3] => 49
            [4] => 70
            [5] => 80
        )

    [2] => Array
        (
            [0] => 20
            [1] => 30
            [2] => 40
        )

    [3] => Array
        (
            [0] => 47
            [1] => 57
            [2] => 67
        )

    [4] => Array
        (
            [0] => 84
            [1] => 85
            [2] => 86
            [3] => 87
        )

)

and after json_encode():

[[19,44,69,99],[26,27,39,49,70,80],[20,30,40],[47,57,67],[84,85,86,87]]

But! If i use unset() when she is after json_decode() (somthing like this)

unset($Array[0][0]);

json_encode() will be return this:

[{"1":44,"2":69,"3":99},[26,27,39,49,70,80],[20,30,40],[47,57,67],[84,85,86,87]]

Change [] to {} why it happens and how to prevent it?