By default NXPowerLite will not reduce a file unless it has changed by +/- 64KB or the optimization settings used are more aggressive than the original settings used. 'More aggressive' could mean lower image quality, smaller resolution or optimization of hidden data elements.
The reason behind this feature is that NXPowerLite needs to ensure that files are not over-optimized. This can create pixelated images and unusable content. So NXPowerLite leaves a small amount of metadata in each file that is successfully processed. It can then decide what to do and apply the rules outlined earlier.