Check this out.
During the period of this camp's existence 405,222 prisoners -- men, women and children -- were entered in the books. Of this number, around 340,000 persons perished at Auschwitz and in the other camps.
There seems to have been some sort of plaque at Auschwitz referring to (only) 340,000 people perishing there.
Now, as (rudely) noted by a Sergey Romanov, (I think he was a poster here at one time), the plaque seems
to refer to registered
("...were entered in the books. ...") inmates.
Now, I must admit, that did seem to be a possible loophole. However, Irving's comment seems dead on. That is, why would any plaque refer only to the deaths of registered inmates? Why not everyone who died, registered or otherwise?