Question: What distinction about his birth makes the contemporary of Bill Pollack unique as a winner of the Formula One World Drivers' Championship?

Context:
2015 FIA Formula One World Championship
The 2015 FIA Formula One World Championship was the 69th season of FIA Formula One motor racing.
 It featured the 2015 Formula One World Championship, a motor racing championship for Formula One cars, recognised by the sport's governing body, the Fédération Internationale de l'Automobile (FIA), as the highest class of competition for open-wheel racing cars.
 Twenty-two drivers representing ten teams contested nineteen Grands Prix, starting in Australia on 15 March and ending in Abu Dhabi on 29 November as they competed for the World Drivers' and World Constructors' championships.
Phil Hill
Philip Toll Hill Jr. (April 20, 1927 – August 28, 2008) was an American automobile racer and the only American-born driver to win the Formula One World Drivers' Championship (Mario Andretti, an Italian American driver, won the World Drivers' Championship in 1978, but was not born in the United States).
 He also scored three wins at each of the 24 Hours of Le Mans and 12 Hours of Sebring sports car races.
2016 FIA Formula One World Championship
The 2016 FIA Formula One World Championship was the 70th season of the Fédération Internationale de l'Automobile (FIA)'s Formula One motor racing.
 It featured the 67th Formula One World Championship, a motor racing championship for Formula One cars which is recognised by the sport's governing body, the FIA, as the highest class of competition for open-wheel racing cars.
 Teams and drivers took part in twenty-one Grands Prix—making for the longest season in the sport's history—starting in Australia on 20 March and finishing in Abu Dhabi on 27 November as they competed for the World Drivers' and World Constructors' championships.
2019 FIA Formula One World Championship
The 2019 FIA Formula One World Championship is a planned motor racing championship for Formula One cars which would be recognised by the governing body of international motorsport, the Fédération Internationale de l'Automobile (FIA), as the highest class of competition for open-wheel racing cars.
 It would be the 70th Formula One World Championship and the 73rd season of Formula One motor racing.
 Teams and drivers would be competing for the World Constructors' and World Drivers' championships.
 2019 is also scheduled to see the one thousandth World Championship Grand Prix.
Bill Pollack
Bill Pollack was a competitor in the early days of the post World War II California sports car culture.
 Starting his racing career in 1950, he would go on to win the Pebble Beach in an Allard J2 that was owned by Tom Carstens'.
 Bill also won at other California venues of that time: Pebble Beach, Golden Gate Park, Reno, Torrey Pines, Stockton, Madera, Willow Springs, Palm Springs, and the Santa Barbara road races.
 He was a contemporary of Phil Hill, who went on to become the first and only U.S. born world grand prix champion.
1987 FIA Formula One World Championship
The 1987 FIA Formula One World Championship was the 41st season of FIA Formula One motor racing.
 It featured the 1987 Formula One World Championship for Drivers and the 1987 Formula One World Championship for Constructors, both of which commenced on 12 April 1987 and ended on 15 November after sixteen races.
 The World Championship for Drivers was won by Nelson Piquet, and the World Championship for Constructors by Williams-Honda.
 The season also encompassed the Jim Clark Trophy and the Colin Chapman Trophy, which were respectively contested by drivers and constructors of Formula One cars powered by naturally aspirated engines.
2017 FIA Formula One World Championship
The 2017 FIA Formula One World Championship is the 71st season of Formula One motor racing.
 It features the 68th Formula One World Championship, a motor racing championship for Formula One cars which is recognised by the sport's governing body, the Fédération Internationale de l'Automobile (FIA), as the highest class of competition for open-wheel racing cars.
 Teams and drivers are competing in twenty Grands Prix—starting in Australia on 26 March and ending in Abu Dhabi on 26 November—for the World Drivers' and World Constructors' championships.
2014 FIA Formula One World Championship
The 2014 FIA Formula One World Championship was the 68th season of FIA Formula One motor racing.
 It featured the 2014 Formula One World Championship, a motor racing championship for Formula One cars, recognised by the sport's governing body, the Fédération Internationale de l'Automobile (FIA), as the highest class of competition for open-wheel racing cars.
 The season started in Australia on 16 March and concluded in Abu Dhabi on 23 November.
 In the nineteen Grands Prix of the season, a total of eleven teams and twenty-four drivers competed for the World Drivers' and World Constructors' championships.
 It was the first Formula One season since to see an accident with ultimately fatal consequences as Jules Bianchi succumbed to the injuries he sustained during the 2014 Japanese Grand Prix.
 He died on 17 July 2015 after spending nine months in a coma following the accident.
2013 FIA Formula One World Championship
The 2013 FIA Formula One World Championship was the 67th season of the FIA Formula One motor racing.
 It featured the 2013 FIA Formula One World Championship which was open to Formula One cars, recognised by the sport's governing body, the Fédération Internationale de l'Automobile (FIA), as the highest class of competition for open-wheel racing cars.
 Eleven teams and twenty-three drivers contest the nineteen Grands Prix that made up the calendar for the 2013 season, with the winning driver being crowned the World Drivers' Champion and the winning team the World Constructors' Champions.
 The season started in Australia on 17 March 2013 and ended in Brazil on 24 November 2013.
2020 FIA Formula One World Championship
The 2020 FIA Formula One World Championship is a planned motor racing championship for Formula One cars which would be recognised by the governing body of international motorsport, the Fédération Internationale de l'Automobile (FIA), as the highest class of competition for open-wheel racing cars.
 It would be the 74th season of Formula One motor racing and would be the 71st Formula One World Championship.
 Teams and drivers would be competing for the World Constructors' and World Drivers' championships.

Answer: American-born