Shawn,
I have little experience in OH-58's, but I do know that there is a difference in rotor rpm in the OH-58A and 206B. Also, if you compare Bell 205 and UH-1H HV's you will see that the 205 HV is set at a higher OGE height (for a 1 second delay) as opposed to the UH-1H (for a 2 second delay). To me this indicates a technique difference, because in a sterile world they should be the same curve.
I still question the validity of the test and the need for the curve. It hasn't stopped loggers, fire fighters or anyone else from operating much of their time square in the middle of the "avoid" areas.