Thanks for checking this out. As I suspected, it is InCD that causes this. Normally I would not care but this causes a problem with software that tracks media based on its serial number.
I am new to Nero so I don't know how receptive they are. I was not encouraged by their initial reply though:
we generate a new disc serial number each time a disc is formatted (as part
of the UDF specifications) not each time a disc is inserted in the system.
There may be something else that attributes a specific disc serial number each time a disc is inserted in the system, but I do not know what.