Page 5 of the manual states: RANGING ACCURACY The ranging accuracy of the Scout™1000 is plus or minus one yard / meter under most circumstances. The maximum range of the instrument depends on the reflectivity of the target. The maximum distance for most objects is 650 yards / 594 meters while for highly reflective objects the maximum is 1000 yards / 914 meters. Note: You will get both longer and shorter maximum distances depending on the reflective properties of the particular target and the environmental conditions at the time the distance of an object is being measured. The color, surface finish, size and shape of the target all affect reflectivity and range. The brighter the color, the longer the range. White is highly reflective, for example, and allows longer ranges than the color black, which is the least reflective color. A shiny finish provides more range than a dull one. A small target is more difficult to range than a larger target. The angle to the target also has an effect. Shooting to a target at a 90 degree angle (where the target surface is perpendicular to the flight path of the emitted energy pulses) provides good range while a steep angle on the other hand, provides limited ranging. In addition, lighting conditions (e.g. the amount of sunlight) will affect the ranging capabilities of the unit. The less light (e.g. overcast skies) the farther the unit's maximum range will be. Conversely, very sunny days will decrease the unit's maximum range.
The difference you are experiencing (20-19.2=.8 yards) is within the device ranging accuracy of 1 yard, and is in fact an error of only 4%. That is pretty accurate
In case you have lost your manual, you can download from here:
Good luck with your device.