Definition
Wet stock variance is the difference between the fuel your records say you should have and the fuel actually in the tanks.
It is worked out per tank as: opening inventory plus deliveries minus sales, compared with the closing reading. A negative result means fuel is missing, a positive result means more than expected.
Small differences are normal because of temperature, measurement and meter calibration. What matters is the trend. A variance that keeps growing, or that repeats on one tank, deserves investigation. Your state tank regulator may also set limits for leak detection records.