浏览器再次使用新选项卡请求同一页面时出现意外行为

I have seen an unexpected behavior when i request my test.php with 2 tabs.

Here is test.php script

if (mkdir(/path/to/directory))
{
 sleep(10);
 rmdir(/path/to/directory);
 echo 'true';
}
 else
{
 echo 'false';
}

As you can see when i request page in first tab its running for 10 seconds sleep while this page running i request test.php again in another tab as expected it has to echo false but this request also running for 10 seconds sleep.

But when i'm doing first request in Chrome and second with Firefox it's working as expected.

I'm testing this with Chrome and Firefox.

Why browsers have this strange behavior?