I don't know about Auschwitz -- but my Uncle fought with Patton and liberated several other concentration camps. His recollections matched up with historical accounts of mass genocide. He said he would never forget what they found. He was just a regular G.I. and not someone high up the command ladder who would have reason to fabricate. These were just stories passed down to family members when he talked about his war time experiences.

My guess is the Red Cross was only shown the part of the camps the Nazis wanted them to see.