I have observed two different kinds of revisionists.
The first kind are simply Jew haters. After they say that they don't believe that the Nazis tried to kill all of the Jews, they either imply or say outright that they should have.
The second kind has an Asperger Syndrome like fixation on details. If the true number was 5.4 million or 6.5 million Jewish people killed, they take exception to the "6 million" figure.