That's because the behavior of the two differ in that aspect:

<?php
$arr1 = array("a"=>1);
$arr2 = array("a"=>2);
print_r(array_merge($arr1,$arr2));
print_r(array_merge_recursive($arr1,$arr2));

gives

Array
(
    [a] => 2
)
Array
(
    [a] => Array
        (
            [0] => 1
            [1] => 2
        )

)

This is clearly explained in the docs for array_merge_recursive, including the example.