I have heard that the Mustang reads a little high, Superflow reads a little low, just make sure you have corrected numbers for comparison. Try to get boost readings logged if possible.
Dyno's IMO are good for comparison of point A and point B. Or as a tool to make adjustments. There are so many factors that impact results from one day to the next, it is always best IMO to make the changes on the same day without the car ever being unstrapped if possible. I have been super happy one day and very depressed a month later on the same tune and hardware. All the while, the cars ran fine.
That being said, because one reads low and one reads high these are only by 1-4% in my experience. The goal of the dyno is to verify a change or establish a base.
Ambient Temp
Eng Temp
Air Flow
Air Flow Temp
Humidity
Altitude
Tire Pressure
Tire Size
Gear Ratio
Clam Open / Closed
IAT at the pull
All of the above and likely more can alter results.