ArcGIS Explorer altitude is the same as Eye altitude for Google Earth.
Google's Eye altitude shows a sensitivity to data altitude and changes with the moving of the mouse. This is similar to using the Measure tool in ArcGIS Explorer.
The returned elevation value is only as good as the elevation provided in the map. The default for ESRI maps is a blend of 90M and 1KM DEM (Digital Elevation Model) for the world. Values are more accurate for data on local drive or data in ArcSDE.