It almost seems like you have a light leak and the right hand image has the bias set to a point where it did not see the low light detail, or taken when the leak does not show up.
Assuming you are using a camera with a M42 hole, you might try to cap the camera with a metal M42 cap when you take a dark frame. Black plastic caps don't always work, since some of them lets through low level IR. I have successfully taken dark frames with the black caps with the red plastic gasket that ZWO provides with their Pro cameras (but inside a refrigerator, so there is very little light to start with).
If a capped camera does not show a light leak, and the left hand image was taken with the camera mounted on your regular imaging train, the same light leak will give you all sorts of problems when taking light frames (for example, leaking differently as the OTA changes orientation during the night, or Moonlight, or the neighbors turning on and off lights, etc). A leak often comes from a focuser, a filter tray, a filter wheel, a camera rotator, or a camera tilt plate that is not completely light sealed, for example. Black electrical tape often helps. The Starizona filter tray for example, leaks light like crazy.
Fortunately, you don't have to waste dark hours. The best time to look for light leaks is in the daytime :-).
Good luck,
Chen
