Reflectance Jump Between Two Instruments

[FCarotenuto]

I've been working on the atmospheric correction of a flight that carried both HySpex VNIR1800 and SWIR384 sensors. In order to optimize the atmospheric correction I've used the SPECTRA module taking as reference a series of spectra that were taken on the ground simultaneously to the flight on an area covered by asphalt. This allowed me to get me quite satisfying results when considering the whole spectrum of the two instruments versus the ground reference measurements as it is possible to see from the following figures. 
   

However I found many pixels, even with similar surfaces such as asphalt, where a massive jump in reflectance between the two instruments was clearly visible. The following image shows the VNIR and SWIR spectra for another asphalt pixel in the flight. The top plots show the at-sensor radiance and the bottom plot the reflectance outputted by ATCOR. While radiances appear to be quite overlapped, the reflectance shows a massive jump. 

   

Any clue at what could be causing this?