el dood isn't a mechanic (neither am i though; we're both enthusiasts)
and i think most people who are aware that a car has a performance figure know its not 'there' car thats being quoted
and if were getting stupidly technical even the aston martin figures are wrong because it takes about 5000 miles to free the engine to max power....no one, no matter how geeky, is going to buy an aston thats been 'run in to max power' and has 5000 miles on the clock....like the gt-r comes with a bhp figure as an estimate; because every engine is handmade. well...every engine is different but when were talking 2% difference on a 155bhp (2.5bhp) it isnt quite the same as 2% of 500bhp (10bhp)
and like anything else the manufacturer is going to quote the best figure it can (unless its swindling the insurance industry as ford did with the fiesta zetec-s).....that best figure IS inside a laboratory on a bench under controlled and repeatable conditions not out on a drive in the country or in a garage with a rolling road at different altitudes, atmospheric pressures, temperatures, humidities, air flow, particulates.....etc....etc....etc