Sounds exactly as it should be. Trying using Pa which have 5 significant figures. You will never see the variations at one decimal in lbf/sq.in. The mass or volume flow rates on the fans might be more enlightening.
Does the fan create the pressure or does the pressure drive the fan? They are linked! And you can prove it to yourself. Put a fan in a huge enclosure. You will have zero pressure difference and the fan will go at full speed. Now put a plate in front and back close to the fan. Run it, the fan will barely be able to push any flow.
Both of bills suggestions are good ones.
I would recommend setting up a non nda assy that you can post. It will help you isolate if your problem is file or setup specific.
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