There are many reasons why a distance calculation could be erroneous, with GPS accuracy issues being the prime candidate. A discussion regarding GPS Accuracy is available on the official US government GPS website.
A caveat though is that 'to be wrong', you need to know the true value, and if you are relying on the distance provided by another service, that may be inaccurate itself.
If you believe the value within TrainAsONE is wrong, and would like to correct, you can do so by editing the activity and entering the correct value. However, be aware that this invalidates detailed activity data (e.g. velocity) and so reduces analysis of the run. As such we would suggest that if the velocity chart looks good, that overriding the distance is not always the best option.