from the knowledge ive attained from dyno's and dyno runs, i can say this. There are two main types of dyno's in use today. the Mustang Dyno and the Dynojet. The mustang dyno gives MUCH more conservative numbers but it simulates real-world conditions from what iv'e heard. Dynojets tend to spit out higher numbers. MOST people use dyno jets, but the hardcore purists in my area look for people with mustang dyno's.
From the numbers you said you got, i wouldnt be surprised if it was a mustang dyno..for a car that gets around 280 on a dynojet, the same car on a mustang dyno gets around 230'ish. hope it helps.
2001 Mustang Bullitt #2379 5.0 stroker, longtubes, other assorted goodies.