PHP为什么这会返回一个多维数组

这是我的代码：

 foreach ($states as $staterow) {
            $state_set[] = array ('id' =>$staterow['id'],
                                      'st' =>$staterow['st'],
                                          'state' =>$staterow['state']);                          
        }
    foreach ($showresult as $display) {
        $display_result[] = array ( 'id' => $display['id'],
                        'name' => $display['name'],
                        'club_email' => $display['club_email'],
                        'description' => $display['description'],
                        'city' => $display['city'],
                        'state' => $display['state'],
                        'url' => $display['url'],
                        'facebook' => $display['facebook']);
                         unset($dispay_result); 
        $statearray[] = array ( 'state' => $display['state'] ); 
        }
    foreach ($statearray as $display) {
        $letterprocess[] = substr($display['state'], 0, 1);     
        }

    foreach ($fulllist as $x => $x_value) { 
        foreach ($letterprocess as $y => $y_value) {
            IF ($x_value == $y_value) {
                $true[] = array ($x_value); 
            }
        }
    }
    var_dump($true);

为什么$true是一个多维数组？为什么它不作为一个简单的数组出现呢。如果我在循环中回显$x_value，它是一个变量，而不是数组。

这是var_dump:

array(12) {
  [0]=> array(1) {
    [0]=> string(1) "A"
  }
  [1]=> array(1) {
    [0]=> string(1) "C"
  }
  [2]=> array(1) {
    [0]=> string(1) "C"
  }
  [3]=> array(1) {
    [0]=> string(1) "I"
  }
  [4]=> array(1) {
    [0]=> string(1) "I"
  }
  [5]=> array(1) {
    [0]=> string(1) "M"
  }
  [6]=> array(1) {
    [0]=> string(1) "M"
  }
  [7]=> array(1) {
    [0]=> string(1) "M"
  }
  [8]=> array(1) {
    [0]=> string(1) "M"
  }
  [9]=> array(1) {
    [0]=> string(1) "M"
  }
  [10]=> array(1) {
    [0]=> string(1) "N"
  }
  [11]=> array(1) {
    [0]=> string(1) "V"
  }
}