As a workaround we can add:
$sessionstats[$status->id] = 0;
on line 891 renderer.php
889 foreach($reportdata->statuses as $status) {
890 $statsoutput .= "$status->description:".$sessionstats[$status->id]." <br/>";
891 $sessionstats[$status->id] = 0; // avoid summing previous sessions
892 }
But please can we have a confirmation if this is a bug or some kind of feature?