Comment 7 for bug 1225514

not sure about the specific, but adding "us" is something gnome-settings-daemon is doing to solve the issue of "standard keybinding should work across keymaps"

the way GTK does it, is that looks through other configured keymaps if the key combinaison exists (e.g if you use u.s and chinese layouts and activate chinese, you want ctrl-C to keep working, for that to work we need "us" to be actively configured as well)

see https://git.gnome.org/browse/gnome-settings-daemon/tree/plugins/keyboard/gsd-keyboard-manager.c#n750