UnderneathTheRadar,
Whilst the QNH is not known for this discussion, idg is quite right in saying that it is typically in the region of 1020+ hPa in Northern China at this time of year.
At the fairly high levels that you describe, even small Temperature deviations from standard will usually have a much larger effect upon Indicated Vs True Altitude, than will QNH. A QNH of 1020 hPa would only account for 184 feet (56 M) of error.
For 37,500 feet Indicated Altitude to translate to 38,500 feet True Altitude would only require the Mean Temperature of the column of air beneath the aircraft to be 6.7°C above standard. This would not be unusual in winter conditions with a low Tropopause, leading to Temperatures significantly above standard above the Tropopause, even though they may be colder at much lower levels.
Regards,
Old Smokey