flock does not block

Question

Please explain why the code below blocks first execution (1607061065) and does not block the second one (1664361639) (the output is attached):

<?php
$fileName = "file.txt";

$fp = fopen($fileName, "a");
if (flock($fp, LOCK_EX | LOCK_NB)) {
    $runId = rand();
    fwrite(
        $fp,
        "processing task [runId]: $runId; [pid]: ". getmypid() ." started \n" .
        (new DateTime())->format(DateTime::ATOM) . " \n\n"
    );

    sleep(3);
    flock($fp, LOCK_UN);
    fwrite(
        $fp,
        "processing task [runId]: $runId; [pid]: ". getmypid() ." finished \n" .
        (new DateTime())->format(DateTime::ATOM) . " \n\n"
    );
    fclose($fp);
    exit;
}
fwrite(
    $fp,
    "script is busy [pid]: ". getmypid() ." \n" .
    (new DateTime())->format(DateTime::ATOM) . " \n\n"
);

I am running this via ab utility which is able to send request concurrently:

ab -c 3 -n 8 http://localhost/

(it is run by 3 instances in parallel 8 times)

And this is what I have in file.txt after execution is finished:

processing task [runId]: 1607061065; [pid]: 2507 started
2022-12-22T17:03:22+00:00

processing task [runId]: 1607061065; [pid]: 2507 finished
2022-12-22T17:03:25+00:00

processing task [runId]: 1664361639; [pid]: 2503 started
2022-12-22T17:03:25+00:00

script is busy [pid]: 2507
2022-12-22T17:03:25+00:00

script is busy [pid]: 2502
2022-12-22T17:03:25+00:00

script is busy [pid]: 2502
2022-12-22T17:03:25+00:00

script is busy [pid]: 2507
2022-12-22T17:03:25+00:00

script is busy [pid]: 2502
2022-12-22T17:03:25+00:00

script is busy [pid]: 2507
2022-12-22T17:03:25+00:00

processing task [runId]: 1664361639; [pid]: 2503 finished
2022-12-22T17:03:28+00:00

Why 1607061065 blocks the first execution and why there is a second process exist which acuires lock 1664361639 which does not block the execution.

Thanks in advance