I'm not quite sure that one tally from one Red Cross survey counteracts millions and millions of people who just up and disappeared never to be seen or heard from again, the photos, the documentation from the Nazis, the films, the eyewitness accounts, etc. etc. etc.
I find it far more plausible that, at the time, the idea of killing 6 million people was somewhat unfathomable...leading agencies to underestimate...and when all the tallies were taken into consideration...the number grew considerably.