If your image is not a perfect rectangle, there is always additional background pixels when subsetting via an ROI. Images must always be rectangles with a specified number of samples and lines while ROIs can be any shape. So ENVI makes the smallest rectangle around your ROI boundary. What you can do is use the 'mask pixels outside of ROI' option from the Subset Data via ROIs tool, and set the background value you want to use for those areas.