Here is part of the deal, most dyno have a "weather station" module to allow the system to correct to sea level which is what SAE Corrected is when it is selected. The problem comes from where they keep it, if the keep it clean and in free air or in a warn cabinet on a cold day etc.
This information is saved in the dyno files, so you can set the corrections.
Something else you have to watch on a dyno is heat soak or an overly cold run will skew a set and give false numbers.
Think of it this way. At sea level I run 12.7's at -1500 Da i run 12.0's I make 50-60 more Hp in good air.
The differences at tracks are things like rotating mass, sprung mass, weight, traction, rolling resistance and mechanical loss are all going to effect runs also how you stage the car. Cold oil will rob you silly. heat soak will do the same.
Guys that have a plan, and stick to it tend to be very consistent because they don't break the routine and have found what works for their combos.
It took 150 passes to get mine sorted out. Playing will coolant temps, oil temps, trans temps and leave RPM's, staging, and shift points.
The biggest mistake I see people consistently do it to buy stuff install it and spend no time logging, dealing with the tuners and getting the combo right. If it isn't laying down what someone else is running with the same combo they think they have crap parts when its a matter of a tune, or running the car right.
Dynos suck
they are a tool if used right for seeing changes and getting run data when you can't get to a track.
-Robert