Different value for Area between Raster Zonal Geometry and Calculate Polygon Area?

Discussion created by gilmanj on Dec 10, 2012
I have a raster as an output from a weighted layer model, the raster has 10 categories of results.  I'm trying to determine the total area of the raster as well as the areas of the different categories.  I used Zonal Geometry to Table and got a nice table showing the areas for each category and I added them all together to get the total area of the raster.  I thought I was finished at this point until a workmate told me the numbers seemed high.  So I thought I'd check the area of the polygon that I used as the processing extent in the model (which should now have the same area as the raster), and to my suprise I get two very different results.

The zonal geometry gives me a result of 798,672 square kilometers
The area of the polygon gives me 592,617 square kilometers

What is happening here?  Is there that much difference in how the two methods calculate area and thier potential errors?  I though that maybe the difference came because the raster is using full cells to calculate the area and the polygon would be more "rounded", but the difference between the two results is quite large.  What am I missing?

Thanks in advance to anyone that replies.