ICE displaying 100+ mpg?

I drive an ICE (gasoline) smart car. In stop-and-go city driving under 45 miles per hour, the vehicle's display is hitting 100+ mpg at times throughout the city drive. How does the smart calculate the 100+ mpg? Does the spike occur when I remove my foot from the gas pedal upon approaching a red light? I put the 5-speed manual transmission into neutral for only about the last 20 feet or so before the stop.