How to accurately measure CFM with Digital Anemometer and Duct
When I worked in a clean room we had to certify the flow hoods and the flow in the pipes. With pipes  we had to measure multiple locations and average the readings close to the outside ends up being slower flow than 1/3 way to the center. On a square plenums we had to take 9-12 reading depending on size. Prior to doing this many times I would have thought the outside was the fastest but the drag of the sides slows the air down, in the center close to the fan there is very little flow unless squirrel cage blower. So try taking multiple measurements and average the results it may be more accurate.