It’s the World Almanac they quote and, it gets its figures from the American Jewish Committee (AJC.)

Pre war the numbers in both the Almanac and AJC say 9,739,200 Jews lived in Europe

In 1946 (as there was no census made from 1940–1945 due to the war) the AJC stared that the Jewish population of Europe had been reduced to 3,642,100. The Almanac did not pick up on this until years later.

http://ajcarchives.org/AJC_DATA/Files/1946_1947_13_Statistics.pdf

In the meantime, the inaccurate non updated version of the World Almanac was touted to be the gospel despite the fact that it had yet to go to it’s normal source to get the correct figures. This slight of hand by Deniers is typical of the dishonest examination of facts they are well known for. Other example number in the hundreds or more ranging from the similar Red Cross misrepresentation to the idiocy of the Auschwitz swimming pool.

https://www.quora.com/Is-Jewish-World-Almanac-legit-Its-quoted-by-holocaust-deniers