if you expect no subsidence at this location it means that your subsidence map is 208 mm too high. If there was no change at this pixel there should be a zero. After unwrapping your product a product of relative changes within the raster, given as absolute values (mm in this case if you applied the formula correctly).
So make your raster 208 smaller and you should theoretically have the absolute height changes during the two overpasses.
However, there’s a strong ramp in your image wich distorts the result massively. I don’t think you did anything wrong so far but maybe there’s too much time between your images.