It was reported early on in this thread that the QNH at the time of the crash was 1027hPa. Since the SBS altitude data is related to standard pressure, it was under-reporting the altitude by 14 times 27feet, i.e. 378 feet.
My understanding is that the ADS-B data originated from a RadarBox, not an SBS.
This means that we can't be sure whether the altitude readouts were relative to 1013mb, or were corrected for the prevailing QNH (RadarBox will do the latter automatically, but only if the appropriate box is ticked).