strange bahavior of \DateInterval::format("%a:)

MotoTripExtensionTest.php

       $now = new \DateTime();
        $before = new \DateTime('-2 days');

        $intervalFromDiff = $before->diff($now);

        $intervalFromConstruct= new \DateInterval("P2D");
        $intervalFromString= \DateInterval::createFromDateString("2 days");
        echo "\n a:".$intervalFromString->format("%a");
        echo "\n a:".$intervalFromDiff->format("%a");
        echo "\n a:".$intervalFromConstruct->format("%a");