The way they test fans is the same way they test any consumer product.
Under the conditions that will allow it to perform best.

It is a fair basis for comparison. As well flow is reduced when ever you do anything. If you bend the tube at 90 degrees: flow reduces by 60%

There are ways to keep for flow up, big ducts help a lot. It’s an exponential curve. 8inch is Way probably twice as good as 6”