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If you saved the NITF to jpg, then you must have saved it from a display window. When you save an image from the display window, you are saving the image just how it looks in that display window which is a streched, byte-scaled image. ENVI has a setting in the Preferences that determines what sort of stretch is applied to data when it is displayed. The default is a 2% linear stretch. So, your resulting jpg is likely the result of a 2% linear stretch, byte-scaled from 0-255. If you save the file from the main menu via File ->Save File as, the image will not stretched or byte-scaled, so the values would not change.
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