VATSIM is explained here: VATSIM / IVAO / PILOTEDGE Users - Be aware of an important bug! - #363 by BTVPilot
Note that clients like VATSPY and the VATSIM web page are different than what is shown to the ATC controllers. These clients appear to use true altitude all the time and will always be “off,” unless the pressure and pressure gradient are exactly as per the standard atmosphere model.