This does look a little strange initially, but I think the various planes are presented in the order of the axis that you would be "looking down" to see that plane. I took a DatumCsys apart using the wrapper method UFModl.AskDatumCsysComponents(), and then cast the tags to NXObjects and reported their JournalIdentifiers:
Type: DATUM_CSYS 41828 HANDLE R-739 41816 HANDLE R-740 36650 DATUM_CSYS(0) X axis 36645 DATUM_CSYS(0) Y axis 36640 DATUM_CSYS(0) Z axis 36635 DATUM_CSYS(0) YZ plane 36664 DATUM_CSYS(0) XZ plane 36659 DATUM_CSYS(0) XY plane
The first one in the array of Datum Axes is the YZ plane, which would be the one that you see when you are "looking down" the X axis. You would see the XZ plane down the Y axis, and the XY plane looking down the Z axis.