A raster is added to ArcMap and symbolized using the Classified renderer. By clicking the Classify button on the Classify symbology tab, you can specify the class breaks. Even if you decrease the top break value, dataset values above this are drawn with the symbol for the top class.
This is a known issue with ArcGIS 8.1.
When specifying a class break that is lower than the maximum value of the raster, use the Exclude functionality within the Classified renderer Classify dialog box to exclude values higher than this class break. This will cause all values higher than the value specified for the class break to not draw.