Not knowing how your measure, I assume you are looking at the monitor when measuring. There is no way 5 different mice are measuring even close to each other. Based on measurements of several high profile mice with a 0.005 inch accurate device this is all but impossible.
Maybe the instructions need an update, but the goal of the DPI Analyzer is not to hit 0% deviation. The goal is to move your mouse a set distance without looking at the analyzer, then look at the result. I suspect a lot of users are looking at the deviation and aiming for 0%, not caring about the actual distance moved (which is the most important part). Small adjustments make a big impact here.
If you set i.e. 5 inches with 400 DPI, you need to click and hold the red cross hair, look at your mouse pad (where your presumably have a 5 inch target mapped out), move the mouse the 5 inches indicated on the mouse pad and release. THEN look at the result. If you do even small adjustments with your wrist trying to match 0%, the result will be invalid.