million, the total number of jewish deaths were never lowered
Because we have a better understanding where and when they were killed. We know the names of more than 5m jews who have definitely been killed, so that number is not going down. The initial impression was that most of the killings were done in Auschwitz, but ever since the 1960s we know that a lot of it was done "decentralized". The number in Auschwitz was deemed as incorrect long before it was finally corrected, because Polish governments were keen on it being the main killing site for political reasons.
why numerous censuses show the numbers of jews INCREASED
Because the Nazis kept lowering the bar on what it meant to be a jew. Initially after 1933, being a "half-jew" meant that you faced little discrimination, while it could later mean certain death. People were also increasingly required to prove that they were aryan, and it often tutned out that they had a mother/grandmother who was born jewish, but converted to christianity early in their life and never considered herself jewish.
because it bullshit
Holocaust denial in a nutshell. There is no way to explain hundred thousands of documents, so they just pretend that they dont exist. Fun fact, I am currently researching the history of a small science society, and thus looking into documents that have not been read for decades. There is a massage among them, informing the society that one of their half-jewish members (who was baptized as a baby, and lived 70 years considering herself a good catholic) died during "the evacuation to the East". In truth she was deported to Auschwitz and died there. There is no way anyone could or would have faked this.
can't even count their own population
This is hardly surprising, directly after the war when official bureaus were often overworked, understaffed and possibly partly destroyed. Trying to find out the correct number killed is an ardenous task, and requires researching millions of documents. Which is why numbers changed, but successive generations of historians have done a good job by narrowing it down. Which is why no one can lower the number below 5.1, since this is the number of victims whose name we have. If you want to lower it, you must explain what happened to the others.