dyno's have a correction factor feature which will adjust the power to show as if the engine was running in perfect temp (19C) air pressure etc etc. power could be actually as much as 10% less.
a mate had dyno sheet for his 302W Cortina, it showed like 520hp (3100lbs) right at the bottom of the dyno sheet it said, power was within +/- 10%
that is way too much to be off.