The hard-coded output raster resampling ratio used by ArcMap Server was not set to 'best'. This setting controls the downsampling of raster layers in the map. When set to a value less than 5, or 'best', raster layers in the map are downsampled to a courser resolution at time of output. This setting affects both raster and vector export formats.
The raster resampling ratio is hard-coded within the ArcMap Server code. It cannot be changed by users.