The "1979-latest" file published by DfT is cumulative: it already
contains every year of data up to the latest release, including recent
years that also have their own individual-year files. Because of this,
any request that includes a year before 2021 is satisfied entirely by
that one file; individual-year files are only added when all requested
years are 2021 or later, to avoid downloading and duplicating rows that
are already present in the 1979-latest file.