It seems like they measured how much you bought, and then calculated how much you wasted based on a combination of weight, BMI, and metabolism.
The number would be less surprising to me if it included how much stores and restaurants waste, as I could see them throwing out expired food. (I've read a good amount give it to homeless shelters before it'd expire though.)
$1500 a year could be quite useful. This money could be used to get food to the hungry. The family could also spend about $25 more a week on healthier (or more sustainable) food.