1 is clearly an offset. The reason for using it is unknown, it could depend on the necessity to alline the reading with other device

1250 is a coefficient for converting the ADC reading into volts. The actual coefficient should be 12.5 but due to the integer math limitation the author did multiply it by 100 to obtain 1250 and then he devide the product by 100.

All these numbers are related to a specific application, so you should ignore them, since very likely, your application will require different numbers as coefficient and eventually an offset.

Cheers

Al.