My thinking was a bit backwards. Mustangs don't really read lower, they seem to be more realistic (in dyno worlds) and I believe it was the Dynojets that were a bit looser with the numbers.
With that said, calibrations have EVERYTHING to do with how well a dyno does, along with operating conditions. They'll differ from run to run and day to day and between dynos of the same make. I'll go along with Powerfreak though in that no matter what dyno you use, use the same one every time for checking mods to get a realistic delta on the numbers.
2010 2SS/RS Auto, Sunroof, Engine Cover. Transformer Edition package.
2013 ZL1 ragtop #5478. 1 of 54 CRT M6 'verts.