Then I stand corrected.
I was in an ICAO meeting in the early 1990's where all this was explained, the earth being a " potatoid" (I remember that term !) therefore every major State used a different reference point/system fitting their Country or continent surface. Since GPS was going to be used worldwide , the USA GPS standard was chosen to the the ICAO standard for their GNSS. ( The then USSR fiercely opposed this , but since they dissolved around that time , they lost the argument )
To be fair in those days we were looking at 2D ( Lat/long positions maps, etc..) , not altitude at all.