In simple terms, the guessometer bases its calculation on the last journey. So if it was up a steep hill - i.e. using energy to overcome gravity rather than distance - you will get a low prediction. If, it was downhill - i.e. coasting using none - then you will get a silly high figure.
So you need to use the car over a period of time to understand the driving conditions and how they influence the range. So don't worry too much about the figure immediately after charge. What is important is how far you can actually drive.
BTW Ambient temperature also influences the range figure. So if you are looking at it first thing in the morning after a chilly night it will be lower.