PHP ключ в массиве не заменятся

Почему в первом коде "1" ключ не заменяется, а втором элемент "b" заменяется?

<?php
$inventory = [
    "0" => ["sku" => "81231846523248", "name" => "Spilvens", "price" => "12.99"],
    "1" => ["sku" => "84751530004684", "name" => "Pleds", "price" => "24.99"],
    "2" => ["sku" => "81474113134254", "name" => "Aizkari", "price" => "19.99"],
    "3" => ["sku" => "84554454386186", "name" => "Veļas grozs", "price" => "29.99"],
    ];
$replace_inv = ["0" => ["sku" => "81231846523248", "name" => "Spilvens HOME", "price" => "14.99"]];
$inventory = array_merge($inventory, $replace_inv);
echo $inventory["0"]["name"];
?>
<!-- Второй код -->
<br>
<?php
$a = [
"a" => ["sku" => "81231846523248", "name" => "Spilvens", "price" => "12.99"],
"b" => ["sku" => "81231846523248", "name" => "Spilvens", "price" => "12.99"]
];
$b = ["b" => ["sku" => "81231846523248", "name" => "Spilvens HOME", "price" => "14.99"]];
$a = array_merge($a, $b);
echo $a["b"]["name"];   
?>'

Отличие от "подобного" вопроса - он блин другой.