The G40 sight glass serves as a guide and general indicator for volume.  Factors such as air bubbles and variable temperatures can impact the readings reflected in the sight glass and therefore, are not meant to reflect precise measurements.  You can expect a potential margin of error to be +/-1L (0.26 US Gal).

As with any measuring tool, there exists a potential margin for error - from your home measuring spoons to meat thermometers.  The impact of these margins will depend on the process and the tolerance for variability above or below the target value.  In places like laboratories where there are very low tolerances for deviation, glass lab equipment is labelled based on gradings to clearly communicate the resulting accuracy when using the equipment. In analytical processes the final error is then calculated based on the errors of the equipment used.  

On the other hand, in processes like homebrewing, the tolerance for deviation +/- the target value is greater.   As you brew more with your equipment and become more familiar with its performance, you can fine-tune and dial in the process for even more consistency.

With that said, let's take a look at the variables that impact the measurements shown in your G40 sight glass:

When a volume is at different temperatures, the volume will change, either expanding or contracting variably above and below the temperature at which the volume measure is calibrated.  For volumes at 20°C (68°F) (around room temperature), volume readings are considered fairly accurate as this is the typical temperature for calibration. 

So for every 1kg (2.2 lb) of water at 95°C (203°F), the volume that 1L (0.26 US Gal) takes up is 4% greater than the same 1kg (2.2 lb) of water at 20°C (68°F).

As the brewing equipment works with volumes of wort and water at a variety of temperatures, the volumes of this liquid will change and subsequently reflect different measurements at different stages/temperatures during the brewing process.  Due to this expected change in reflected volume, the sight glass is marked in 1L (or 1 US Gal) increments to show a general volume measurement at any given time.

Additionally, when filling your G40 unit there is a chance for air bubbles to get trapped in the recirculation pipework which can influence measurements by up to 0.5L (0.13 US Gal).  To ensure that there are no air bubbles tapped in the pipework, our team recommend running the pump a few times to ensure that any excess air is expelled.