What determines if a "recommends" is installed?

[andy pugh <bodgesoc@gmail.com>]

The package that I am building a live-image to distribute has a list
of recommends:
https://github.com/LinuxCNC/linuxcnc/blob/2.8/debian/control.bottom.in

Live-build installs some of them, but not others:

Recommended packages:
  python-pyqt5-qtwebkit opencv python-gst0.10
The following NEW packages will be installed:
  adduser adwaita-icon-theme apparmor apt apt-utils at-spi2-core binutils
...
  perl-base pigz procps psmisc publicsuffix pyqt5-dev-tools python
  python-cairo python-configobj python-dbus python-dbus.mainloop.pyqt5
  python-enum34 python-espeak python-gi python-glade2 python-gobject-2
  python-gst-1.0 python-gtk2 python-gtkglext1 python-gtksourceview2
  python-minimal python-numpy python-olefile python-opengl python-pil
  python-pil.imagetk python-pkg-resources python-pyqt5 python-pyqt5.qsci
  python-pyqt5.qtopengl python-pyqt5.qtsvg python-serial python-sip python-six
  python-tk python-xlib python-yapps python2 python2-minimal python2.7
  python2.7-minimal python3 python3-minimal python3-pyqt5 python3-sip
  python3.7 python3.7-minimal qt5-gtk-platformtheme qttranslations5-l10n
  readline-common
...

I have set  --apt-recommends true (but it is documented to default to
true anyway)

-- 
atp
"A motorcycle is a bicycle with a pandemonium attachment and is
designed for the especial use of mechanical geniuses, daredevils and
lunatics."
— George Fitch, Atlanta Constitution Newspaper, 1912