I agree that pressure makes a large diference when flowing heads, that is why 10" or 28" is specified when flow data is given.
However, a large % change will be seen when using a large bore size adapter.
What is the point of knowing the airflow the cylinder head flows into a 4" bore adapter,
when the engine bore size is 3.13"?
The head porter knows that a large bore adapter will show a larger head flow number.
I will do some testing and post the results of a large bore size adapter vs. standard bore size.
The head porter is not lying about the results - just making sure the highest flow numbers
are seen. That is what usually sells head work.