php - 关于array_diff_uassoc的疑问

源码：

<?php
  $i = 1;
  function key_compare_func($a, $b)
  {
      global $i;
      var_dump($a);
      var_dump($b);
      echo "=====$i".PHP_EOL;
      if ($a === $b) {
          return 0;
      }
      $i += 1;
      return ($a > $b)? 1:-1;
  }

  $array1 = array("a" => "green", "b" => "brown", "c" => "blue", "red");
  $array2 = array("a" => "green", "yellow", "red");
  $result = array_diff_uassoc($array1, $array2, "key_compare_func");
  print_r($result);

结果：

string(1) "b"
string(1) "a"
=====1
string(1) "b"
string(1) "c"
=====2
int(0)
string(1) "b"
=====3
string(1) "a"
int(0)
=====4
int(0)
string(1) "a"
=====5
int(1)
int(0)
=====6
string(1) "a"
int(0)
=====7
string(1) "a"
string(1) "a"
=====8
int(0)
string(1) "a"
=====8
int(0)
int(0)
=====9
string(1) "b"
string(1) "a"
=====9
string(1) "b"
int(0)
=====10
string(1) "b"
int(1)
=====11
string(1) "c"
string(1) "a"
=====12
string(1) "c"
int(0)
=====13
string(1) "c"
int(1)
=====14
Array
(
    [b] => brown
    [c] => blue
    [0] => red
)

请问：

对于这个函数，他的比较原理是什么？为什么要进行14次比较？