Calibration may be off. The calibration procedure is published in the manual and copied below.
1. Balance the meter in an upright position on a hard surface.
2. Press and hold the START button.
3. With the START button depressed, press and release the ON button. The LCD will count down from 5 to 1.
4. The meter should now read between 5 and 10 P.S.I. (35 - 70 Kpa). This reflects the weight of the meter itself.
Note: During the calibration procedure, care must be taken to ensure that the only force on the load cell is the weight of the meter itself.