An aircraft’s ADS-B transponder transmits the position from the onboard flight computer, which in most modern aircraft comes from GPS data and is very accurate.
If an aircraft flies through a GPS jamming area, the onboard computer can get interfered with and transmit bad position data. In most cases when the aircraft exits the GPS jamming area, ADS-B data resumes with good quality. Still, in some cases, the bad position data will remain for the rest of the flight, which will be visualized as the aircraft flying and landing up to 50 km off course.
On some very old aircraft (for example some older A300, B737, B757, B767, and RJ100) the position is calibrated on the ground before take-off and after that, the current position is calculated based on the speed and direction of the aircraft.
When flying for some time, in wind or in circles, the calculation becomes increasingly worse, which at landing can give position calculations that are up to 10 km incorrect.