random_int generating values less than minimum

I am generating passwords with random_int, but I noticed something weird. Sometimes (purely random), it generates a password less than the minimum value. For example, I set 10 and 15 the limits and once every circa 50-70 tries it pops out a 2-3 character long password. Is there something wrong with my script? It's hard to reproduce the output, just refresh the script till you get a similar result.

<?php 

    $r_number = random_int( 10 , 15 );

    function random_str(
        $length,
        $keyspace = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ,./<>?;:"|[]{}-=_+`~!@#$%^&*'
    ) {
        $str = '';
        $max = mb_strlen($keyspace, '8bit') - 1;
        if ($max < 1) {
            throw new Exception('$keyspace must be at least two characters long');
        }
        for ($i = 0; $i < $length; ++$i) {
            $str .= $keyspace[random_int(0, $max)];
        }
        return $str;
    }

    $password = random_str($r_number);

    echo $password;

?>