The reason for the 1 L (1 US qt) sparge volume difference with the Grainfather app and instructions is that the basic formula printed in our instructions don't take into account if you are using a 220V or 110V version Grainfather.
Because the 110V heater version has less power capacity than the 220V version (1600W vs 2000W), the boil evaporation rate is less (2 L (2.1 US qt) per hour instead of 3 L (3.2 US qt) per hour).
So for a 60 minute boil, with the 110V unit you have to sparge with 1 L (1 US qt) less, because you lose 1 L (1 US qt) less during boil off.
For the more precise results, please see our calculators online or use the app. The printed formula is to show you mostly, how the calculation works.