Mustang dynos are the most accurate, repeatable dynos out there. At least they are for 2wd vehicles. (I don't see how it would be diff for 4wd).
The ones in Calgary at tecmotion and dynomotive read about 13% lower than dynojets in town. However they are much easier to tune on due to their ability to simulate load. I used to love telling people I make 260whp and run low 13s in my honda
IMO it's just a number. What I look for is repeatability for tuning purposes.