OEM mfgs use the SAE J1349 correction factor, plus pretty stringent controls, so their numbers generally are a bit conservative compared to what you would see on an engine or chassis dyno. Many shops will use the J607 correction factor, referred to as standard correction. It uses different temp and baro references. Hp readings are generally about 4 pecent higher. So, you need to see what the weather data was and correction used on your dyno run to be able to get any kind of real comparison. We would almost always see an engine make more HP on an independent shop dyno than the factory rating. Also, a lot of engine dyno testing is done without accessories, mufflers, etc. Then they get chassis tested and see huge losses. When I run an engine on my dyno, I try to have all accessories, PS, alt, etc working, plus complete exhaust system. Then the numbers are more realistic and the tune will be more representative of what you will need in the car.