This!! @gree: Nails it.There are dynos that are calibrated and correct, and then there are dynos that read higher than they should, we refer to them as "happy" dynos here. In the Phx,Az. area you can see a spread of about 15+ HP on a 120 HP build. Mine and 3 of the local dealers that have "Dyno Days" HP Shootouts are within 1-2 HP at the lower level and are considered as conservative, or accurate, depending on your view point. The "Bar Braggers" use the other indys that post big numbers for their "Bragging Rights" customers, but those guys never go to the track or the Shootouts either. Around here in Phoenix, when discussing builds and HP the first question asked is "on whose dyno?" At that point you can decide whether to stop listening....
There are about a dozen common ways to trick an accurate dyno into reading high, or low, whichever you like. I'm not going to get into that because I don't want to educate the "happy dyno" owners with ideas that they didn't think of.
Bottom line. Find a reputable dyno guy, or try others just for a couple of pulls and see if the results are comparable. Asking a lot of questions of the really fast guys in your area, (you know the guys that race), will get you a start. If a fast guy giggles when you ask about a certain shop that has a dyno, you probably have a good start on an answer.
When discussing builds or dyno numbers I tell my customers " I'm not going to tell you what you want to hear, but I will tell you what you need to know." TIMINATOR