Shutter speed. Plain and simple.
Your middle exposures, and even your darkest exposures are not perfectly sharp (too slow shutter).
As far as I can see, you are not using a tripod because the frames are not identical.
Step 1: Use a tripod. This will stop camera shake and all things that are stationary in the pictures should turn out fine. If you have no tripod, try step 2.
Step 2: Raise the ISO (and open the aperture if its not wide open) to get faster shutter speeds. This also includes zoom. If you are zoomed in, the camera will have a smaller aperture (larger F number) and let less light in. Camera shake is worse when zoomed in. So zoom all the way out if you are not already.
Explanation:
This is why you are experiencing what you are seeing:
Cameras need to "catch" a certain amount of light for a correct exposure. When there is a lot of light (during the day), the camera can do this quickly. At night, where there is little light, the camera will take longer to catch the same amount of light.
Your first exposure is the "correct" exposure and catches a certain amount of light, which takes a certain amount of time.
Your +1 exposure is asking the camera to catch TWO TIMES as much light = shutter speed 2 times longer
Your +2 exposure is asking the camera to catch FOUR TIMES as much light = shutter speed 4 times longer
Your +3 exposure is asking the camera to catch EIGHT TIMES as much light = shutter speed 8 times longer
The same thing occurs with your -1 -2 or -3 exposures, only opposite. The shutter speed is 2, 4, or 8 times quicker. This is why the darkest frame will always have the fastest shutter speed, and the brightest frame will have the slowest shutter speed.
In my opinion, you can forget hand-held HDR at night. Shutter speeds are just going to be too slow.
