Request to Referrers API.php returns different result from API Request

I have a custom report in a custom plugin. This report should show the campaign keywords for a specific campaign name. This happens as follows ($kurzform form is the campaign name):

public function getPortalKampagnen($kurzform, $idSite, $period, $date, $segment = false)
    $segment .= "referrerName==".$kurzform;
    $campaigns = \Piwik\Plugins\Referrers\API::getInstance()->getCampaigns($idSite, $period, $date, $segment);
    if($campaigns->getRowsCount()!=0) {
        $result = \Piwik\Plugins\Referrers\API::getInstance()->getKeywordsFromCampaignId($idSite, $period, $date, 1, $segment);
        $result = new DataTable();
    return $result;

public function getHslaKampagnen($idSite, $period, $date, $segment = false)
    return $this->getPortalKampagnen("hsla", $idSite, $period, $date, $segment);

However, I sometimes have a different result (less) than shown in the subtable of the campaign report. The result is also different from an API request with exactly the same information (date, segment etc.).
What am I doing wrong?

I still have this problem. Does anyone have any ideas?