Grocery shopping algorithm, a joke

A fellow engineer told me this joke, and the computer scientist in me found it amusing...
A woman asked her engineering husband to go to the store for her to get a gallon of milk. "And," she added, "if they have eggs, get a dozen."

image by cannoncan (Karen Andrews), via rgbstock.com

When he returned with 12 gallons of milk, she was agast: "What?!?"

"There were eggs," he replied.