This article is from the Holocaust FAQ, by Ken McVay firstname.lastname@example.org with numerous contributions by others.
This is because those lists refer only to those who where assigned
serial numbers. The majority of people transferred to the camp were
classified as "unfit for work" and gassed immediately. This fact is
noted, for instance, in a report written by top SS officer
Franke-Gricksch to Himmler (Get pub/camps/auschwitz/gricksch.rpt;
& ~/auschwitz.faq1, which discusses the registration process). These
people were not registered anywhere, and no one recorded their names.
(Testimony of Polish prisoner Aloiz Oskar Kleta, Shelly, p. 284;
Fertig, 12; Fleming, 174. Also, see testimony of Henryk Tauber,
Pressac, page 488, as to how the SS routinely burned documents
regarding the number of victims.