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Your subframes are offset from each other, while the galaxy is in all of the frames it is not in the same place in the picture. The multiple peaks in the histogram represent the differing levels of background in each section.
In DSS you have a few settings with diagrams which will illustrate your issue. In stacking settings under result there is standard mosaic and intesection.
Unfortunately these settings will not do you much good in this case (as I think the resultant frame from a intesection mode stack would be too limiting) The best course of action is to keep the object in the same spot each picture.
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