https://wiki.gentoo.org/wiki/DISTDIR The default value of DISTDIR variable is in /usr/portage/distfiles. No DISTDIR definition is in make.conf: # emerge --info ... DISTDIR="/usr/portage/distfiles" ...
For new installations, it is /var/cache/distfiles. For existing installs, the old setting was preserved by the portage ebuild, written into /usr/share/portage/config/make.globals so as to not break anyone's old setup. If you set the new value and rebuild portage, it'll update make.globals.
Thanks for clearing this up.