one of our users had the same problem a while ago. If I remember right, one cause could be that the polygons were digitzed in a counter clockwise direction; it's also possible that the polygon features are (self) intersecting. (or a combinationof both?) I think to fix this you will have to run the "Repair Geometry" tool (ArcToolbox -->Data Management Tools -->Features -->Repair Geometry). Select your polygon shp file/feature class as "Input Features". (in our case we also unchecked "Delete Features with Null Geometry") Hope that helps!