Assuming that people who wanted to buy one during Nov-Dec would have purchased one as soon as they were available (which is a logical assumption) then those sales are already accounted for.
Basically it's 4 million minus Switch YTD minus the sum of Nov-Dec 2017 Switch sales divided by remaining weeks. So instead of those sales being in the Nov-Dec 2017 section they're in the Switch YTD section. They still are accounted for in the approximations.
@160rmf Why would you expect the 2nd year of a Nintendo handheld/hybrid to sell the same as the 5th year of a dedicated home console where Sony has long-abandoned Japan as a prime market?