Error: Area and Volume statistics on rasters ignore Z factor
** Fixed at ArcGIS 8.2 **
The Z Factor property listed on the Area and Volume statistics is ignored in the calculation when the input surface is a raster.
This is a known issue.
Solution or Workaround
You can use the ArcGIS 3D Analyst developer sample Raster Math Command to factor the surface before running Area and Volume.
Alternately, if you have ArcGIS Spatial Analyst installed, you can use the Raster Calculator to factor the surface.