It's not all about how many different types of dynos are out there, but more about the environment they are set up in. A dyno set up in south florida will give a much different reading than it would if the exact same one was set up in denver. Temperature, humidity and atmospheric pressure can all drastically change results as much as the calibration can. As has been said before, best way to gauge the approximate gains from any part is to look at the percentage gain between two pulls (before and after install for example) at as close to identical conditions as possible.
Also, this post gives +5hp while remaining 98% fat free... that's how important flat numbers are.