Basically because General Motors bought Opel (an old German firm) and Vauxhall (an old British firm) and thought, wouldn't it be cheaper if they built the same car for each country, but just put the right name on it for the country. So the Opel Kadett is the Vauxhall Astra and the Opel Ascona is the Vauxhall Cavalier. I used to deliver to the Opel engine plant in Vienna (Wein) and the suppliers where I picked up from said they made exactly the same parts for Opel and Vauxhall.