They were done @ Steeda, so they probably know what they are doing. Since other cars were dynoed at the same time you should be able to get comparative #'s and find out what dyno etc.
http://www.mustangworld.com/forums/showthread.php?threadid=236683
There is ALWAYS going to be measurement error, differences due to temperature, humidity, barometric pressure, dyno type, etc. etc etc. I have seen Mach 1 #s as low as 260 RWHP and as high as 285 RWHP, so the average for the Mach 1 looks like its around 270 - 275 RWHP.
Steeda's #s of 272 RWHP are pretty good, but we'll have to see a number of '05s measured to get a feeling if that's low, high or the average.