The idea behind partitioning is for the OS & Programs on "C" with "D" for Data and is done by most Laptop Manufacturers. That's the theory but in practice it never seems to work as "allsorts" finds its way onto the "C" partition. I doubt if the 8GB you see on the "D" partition are the factory restore files are these are normally hidden. Click on your "D" partition to see what data/files it contains.
iMac Pro review