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It's all in your head.
You do something that you think will improve your mpgs and then when you are retesting your mind is concentrated on the improved mpgs - so without realizing it probably driving a bit easier.
Cars are so sensitive to speed and how heavy the foot is - affecting mpgs. Pretty easy to skew the results by driving just a hair easier after the mod.
You see this all the time with different brands of oils. Someone puts in a designer oil, and magically they are getting great mpgs - BS.
For this situation to be true, the one filter would have to be so restrictive that it was negatively impacting mpgs, then adding the second one just brought it back to the baseline of the stock airflow. More likely though just inacurate testing.
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