Sometimes they are, depends how you define The Holocaust. To simplify a bit, the term has at least three slightly differing meanings:

• The Nazi genocide of the Jews, also known as the Shoah.

• All Nazi anti-Semitic policies from 1933-1945, not just killings, so including the ghettoes, the concentration camps (not the same as the death camps), etc.

• All Nazi mass killing policies, so including Poles, Soviets, Serbs, Slovenes, Romani, etc.

It's mostly a matter of definition.

The Holocaust was originally considered as the Nazi attempt to exterminate the Jewish population by various means including concentration camps, ghettos, extermination camps and most famously the gas chambers. Some historians still define the Holocaust to exclusively refer to only the killing of the Jewish population, and this is the common perception of the Holocaust amongst the public.

More recently some historians now define the Holocaust not just by it's Jewish victims, but more principally by the methods used to kill them. This definition then also includes other victims of the camps, ghetto's etc, such as the ethnic Roma, Homosexuals, leftists and some Soviet POW's. Including these already increases the 6 million number quite a bit.

Some historians also include all Nazi crimes under the Holocaust, but the crimes against Slavs and some Soviet POW's were conducted under a different operation, using different infrastructure to the concentration camps etc which were built to kill Jews. Most Slav victims were killed as part of the hunger plan, a component of Generalplan Ost (roughly 'master plan east') in which civilians in occupied countries in eastern European nations and Russia, and soviet POW's were intentionally staved on mass, though there were also a large number of mass killings.

The things that differentiate these sets of killings are the ideological reasoning's, the localisation, and the method/organisation of the killings. Slavs were killed as part of an intention to colonise eastern Europe by a German population (see Generalplan Ost, this subject is a books worth of content on its own), which is different to their reasons for killing Jews (this is a big subject I won't attempt to summarise right here). The extermination of Jews occurred in all German occupied territory (including Germany, though the persecution of Roma was not as universal) while the staving of Slavs was localised to eastern Europe (including what was Soviet territory, but is considered to be part of the European continent) and did not occur in western territories. While finally, as discussed above, the killings were conducted for the most part using different methods and infrastructure (there is a lot of cross over with Soviet POW's, some were integrated into the concentration camps, some were starved to death in dedicated POW camps).

Which Would Almost Triple the 6 Million Number?

The number I always heard was 10 million :6 million Jews, 3 million Gypsies, 1 million other.

A question i have had about the holocaust for a long time is the structuralism vs Intentionalism debate (if it is still a debate). I feel that issue is extremely complicated and both sides make a reasonable case but i am wondering what the consensus view among academic historians is.