The definition of the HP (Horse Power) unit can be confusing and varies depending on the standard used. US uses SAE (Society of Automobile Engineers) standard while Europe uses DIN (Deutsches Institut für Normung) standard. Multiply HP SAE by 1.0139 to get to HP DIN.
Not to mention the BHP (Brake Horse Power) which measures the horsepower of an engine without taking into account the loss due to all the additional components that consume power like alternator, power steering, water pump, etc...