Custom Bootloader on Atmega328p - PU - 28 pin DIP
The Atmega328p has 8 analog inputs. Some packages (such as the DIP) don't have enough pins to bring all of them out, but they are in there. The SMD package brings all of them out, but it apparently the Pro Mini does not pick up two of them. The net result is the same - 6 useable and 2 unusable analog inputs.