I've created 2 user with following code which works fine:

$data = array(
    'code'          => 'USER1',
    'firstName'         => 'Milton',
    'lastName'      => 'Waddams',
    'email'             => '[email blocked]',
    'subscription'  => array(
        'planCode'      => 'FREE'
    )
);
try {
    $response = $client->newCustomer($data);
    echo "\n\tCreated\n";
} catch (Exception $e) {
    echo "\n\t" . 'ERROR: (' . $e->getCode() . ') ' . $e->getMessage() . "\n";
}

$data = array(
            'code'          => 'USER2',
            'firstName'         => 'Albert',
            'lastName'      => 'Einstein',
            'email'             => '[email blocked]',
            'subscription'  => array(
                                     'planCode'         => 'FREE'
                                     )
            );
try {
    $response = $client->newCustomer($data);
    echo "\n\tCreated\n";
} catch (Exception $e) {
    echo "\n\t" . 'ERROR: (' . $e->getCode() . ') ' . $e->getMessage() . "\n";
}

After that I try to delete both users with:

try {
    $response = $client->deleteCustomer('USER1');
    echo "\n\tDeleted\n";
} catch (Exception $e) {
    echo "\n\t" . 'ERROR: (' . $e->getCode() . ') ' . $e->getMessage() . "\n";
}

try {

    $response = $client->deleteCustomer('USER2');
    echo "\n\tDeleted\n";
} catch (Exception $e) {
    echo "\n\t" . 'ERROR: (' . $e->getCode() . ') ' . $e->getMessage() . "\n";
}

First user is deleted succesfully, but on the second one I get a ERROR: (404) Customer not found

Why is that?