The difference maps show the difference between the global horizontal radiation (GHI) and direct normal radiation (DNI) between version 7.1 and 7 and version 7 and version 6.1.
The differences are induced by different time periods (1996-2015 for version 8, 1991-2010 for version 7.1, 1986-2005 for version 7.0 and 1981-2000 for version 6.1) and ground stations and enhanced satellite data.