关于类__unset(),__isset()的问题，__unset()掉的属性，通过__isset()检测会执行两次？

[beaaar]

<?php
class Phone {
private $band;
private $color;
private $name;

function __construct($band = 'nokia', $color = 'black', $name = 'Lumia920') {
$this->band = $band;
$this->color = $color;
$this->name = $name;
}


function __isset($property) {
if( isset($this->$property) ) {
echo $property.'is '.$this->$property.'.';
}else {
echo $property.' is not set.<br/>';
}
}
function __unset($property) {
unset($this->$property);
echo 'Unset '.$property.' successfully.<br/>';
}
}

$phone = new Phone('nokia','blue');
unset($phone->name);
echo '<hr/>';
isset($phone->name);
?>
输出结果
Unset name successfully.
--------------------------------------
name is not set.
name is not set.
以上函数，unset掉一个属性后，isset查询该属性是否存在，则__isset()输出了两次；
如果直接isset一个不存在的属性，就会正常执行，不明白是为什么？