Calculating typical floor fan efficiency or dP? (Spec sheet too good to be true?)

Your confusion seems to be in assigning a dP at all. On the back side of the fan, the air is at 1 atmosphere, and on the front side, the air is at 1 atmosphere. There is no static pressure gain at 6,000 CFM. Instead, you would consider the velocity pressure. This is more challenging:

1. First, take the velocity at each point on the outlet of the fan. These measurement points should be as many as possible for the highest accuracy.
2. Square the velocity, and multiply by the area this point covers.
3. Add all of these together, and divide by the total area.
4. Multiply by the density and divide by 2.
5. Convert to Pascals.
6. Use this to define the efficiency.

As someone who has experienced these fans, a significant fraction of the air at high speeds winds up heading out radially or even slightly backwards. Thus, a different manufacturer may have wound up running such a measurement for CFM and including this additional airflow, whereas the others disregarded it, knowing it was useless vortexing. As such, it may simply be marketing.