According to one source, there were 2.7 million victimsof the Holocaust. Your source is wrong and you should be very skeptical of any information from that source. Between 5 and 6 million Jews were killed in the Final Solution. Additionally millions of non-Jews were killed by the Nazis. The total is quite accurate.
Detailed records do exist, but the demographics can be difficult to understand.
I am not sure that I understand that question myself. Further to the excellent response given to your questions by my colleague, Dr. Richard Green, I would like to touch on a couple of points. It is not quite true that accurate records of Jewish population do not exist. It was this that made it relatively easy for the Nazis to round up Jews in every country. Highly accurate demographic estimates of the Jewish people exist and are available to the general public.