Variance maps should accurately estimate noise in the images

The noise in the reduced images is currently not correctly captured by the variance map. 

For an example, looking at the image 8603 and taking random patch of the image without sources gives variance of around 20. On the other hand, mean value in the variance image is around 36, 37. These results seem to be consistent across the whole image. 

Possible causes are 1. wrong gains, 2. given that there seems to be structure in the noise, wrong estimation of the readout, 3. other options.