Countries presented are based on language already selected
The selection process is laid out in the README for
localechooser (used to build debian-installer images):
The purpose of localechooser is to ask the person doing the
installation about his preferred language and country of "residence".
This information can be used to choose which language and country or
region to use during installation and set this language and country as
the default language and country after the installation.
The country choice question refines the country choice inherited from
the language choice. It allows choosing any "country" in the
world. Depending on the language+country combination, this will be
used for setting a default locale or not (see below for details).
localechooser README (link to git blob), lines 4-26)
And, more specifically:
-always prompt the user (high priority) with a short list of all
countries for which a valid locale exists for this language
THIS LIST IS BUILT AT BUILD TIME for countrychooser, not on the fly
at install time. It is built only for languages which have a debconf
translation for countrychooser
(lines 146-150, ibid)
There is further detail on the process, which goes about selecting a language, location and finally locale.
In your example, I would presume that you have selected English, as those countries you have have a significant English-speaking presence.
credit to pabs and youpi in #debian-boot, who gave a brief overview of the process