Why did you choose to make tea before pressing 'Install'? Seems like the most efficient way to install a MacOS update would be to press 'Install' before you go to make your tea. Maybe Job's sensed your strange decision to make tea before pressing 'Install' and forced an odd change of installer files.. Finder feels snappier though.
Well dah! Because I don't leave an update running unattended on my main work machines in case something goes wrong.