A lot can depend on the type of dyno 'correction' that is applied, Dynojet (On Motorcycles) have one called (from memory) ''DJCHP' thats as well as the SAE, STD, EEC and DIN.
The correction factor on a 'std' ideal day would be '1' and then either side of '1' dependent on ambient conditions of pressure (altitude as well), temperature and Humidity.
With Motorcycles Mr Dynojet got pi**ed off with his 'accurate' dyno always reading lower than his competitors, being honest if two dyno operators were claiming 180hp and 200hp respectively which one would you go to?
So he basiically added a 0.1 into the correction factor to match his competitors, In the Motorcycle world you still see some really chuffed owners proudly showing/posting power curves, but looking at the bottom right you also sometimes still see the 'DJHP correction' which means his 200hp Motorcycle is really about 180hp.
There are that many 'fiddles' that can go on (Yes i've done a fair bit of dyno work
) it's a bit of an eye opener, I have seen in the past MR K&* mess around with a kit for hours until he gets a 'fluke' high reading and say ''That''ll do, print that one off and i'm out of here''