BA's older 767s broadcast ADS-B data that's derived from inertial (gyro) based navigation systems, as opposed to most newer aircraft that get their data from GPS.
Inertial systems suffer from position drift, and the BA 767s seem worse than most.
It's not a problem for ATC, because the aircraft also transmit a parameter (that you can't see on FR24) indicating that the positional data is subject to a large potential error - it just means that they can't fly in the few areas of the world (Hudson Bay, for example) where accurate ADS-B is mandatory.