Why is the tread depth in the report lower than my own measurement?

For safety reasons, the result in the SnapSkan report is based on the most worn tire. The result for individual tires is the mean value for the measured main groove treads rounded to the nearest millimeter. Click the tires in the report to see detailed measurement results for that tire. 

Example: 
Measured tire grooves: 5.5 mm, 4.6 mm, 5.7 mm, 5.4 mm
Mean: 5.45 mm
Rounded value: 5 mm 

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