I'm very interested in this subject and would be glad to hear more..
The thread is drifting a bit but search the Eurocontrol site for Cascade and look up Validation and you'll find loads on the subject. Positional inaccuracies with ADS-B are a daily occurrence but the Eurocontrol data shows that these aircraft are also indicating that the position report is of poor quality. Does the SBS box display the quality indicator ? Unless the aircraft is flying in mandatory ADS-B airspace it is not obliged to transmit accurate reports or be equipped to do so.