If you really look at the typical steel wheel you will see the raised porting and the "coined" raised part the lug fits into.
Look at the hub and you will also see the marks are not flat all around the hub.
The raised area acts as bellville washers to load the fastener and to help keep the clamping force on the wheel.
Almost all steel wheels have this feature since WWII. Very heavy steel wheels with more lugs do not have this since the wheel center is very much thicker.
Look carefully at the back of this wheel.
Here is a cross section drawing where you can see the part more clearly
Without this small offset the wheels would loosen easily and fail.
This was discovered during WW 2 and almost all stamped drop center steel wheels since have this feature