Zonal Statistics results are not equal

Discussion created by pfroemke on May 27, 2011
Latest reply on Jun 14, 2011 by pfroemke
I???m trying to get a value for mean water runoff by wilderness area. I am using the Zonal Statistics as Table tool with wilderness areas as the input raster and water runoff as the input value raster.
Zonal stats produces higher values for total runoff and area when combining wilderness with owners and regions compared to wilderness with just owners.
1. Set environment - projection, extent, cell size (100 meters), and 100-meter snap raster.
2. I start with two versions of wilderness by federal agency polygon shapefiles: one divided by Forest Service regions and one that isn???t.
3. Then I use the Feature to Raster tool for converting the two wilderness shapefiles to rasters.
4. Project and resample the runoff grid from 5 km to 100 meters so that it matches the wilderness rasters.
Zonal Statistics as Table Results:
1. Total wilderness area is greater for wilderness by agency and region compared to wilderness by agency.
2. Total runoff is greater for each wilderness when split by agency and region compared to wilderness by agency.
Does anyone have any idea why I am getting these different results?