Irrigating to maintain a lawn while also reducing water use is a challenging task. Compared to a uniform farm field, a lawn can be a mixture of grass, shrubs, and trees. The house and trees can create shaded and sunny areas that vary by season, slopes, and soil types. All this affects the amount of water that plants use and how much water you need to apply. However, unlike crops, homeowners do not often require maximum production from their lawns. Therefore, the sprinkler runtime adjustment method makes some simplifying assumptions.
Here are the details of the method. From this document: “Current turfgrass studies indicate over-watering is a problem. [This method] assumes 100% watering efficiency, with no losses due to runoff or lack of distribution uniformity. In fact, this is not entirely true. Normal watering run times set only to the current evapotranspiration (ET, which is how much water the lawn is using) and discounting all other factors would result in over-watering. The [method], however, uses irrigation inefficiencies to its advantage. The actual water applied to the soil using the [this method] will be 20% to 30% of adjusted ET by the mere fact that it never accounted for inefficiencies to begin with. This winds up being very close to optimum settings, without the hassle.
The data used to calculate monthly adjustments are from this WSU webpage except for George and Goldendale which use AgriMet Station data.