/u/commiespaceinvader has answered this question in their answer. The long and the short of this was that the Polish and Soviet authorities derived the larger number from some questionable methods. This was not some grand conspiracy to fabricate a Jewish genocide at Auschwitz; the Auschwitz State Museum during the communist period tended to either ignore or be disinterested in the murder of Jews at the site. Warsaw was more interested in portraying Auschwitz as a site of Polish martyrdom. Moreover, most Western scholars of the Holocaust ignored the state museum's inflated numbers and arrived at the more reasonable numbers. This underlay the change in the plaque after communism fell.