Making up performance specs

In the 1950s, 60s and 70s, salesmen often made up HP, economy, top speed ect. The Jaguar E-type Series one and two can produce 225bhp, but they claimed 265bhp. Why did british leyland sell 10s of thousands (if not 100s of thousands) of the worst cars ever built in history for goodness sake?
I would have thought that the game would just spec up the actual performance, but is that the case?
I would have thought that the game would just spec up the actual performance, but is that the case?
