Sentinel-1 fusion problem with Landsat 8

Hi all
I work on the floods: I made a fusion between a picture Radar Sentinel-1 GRDH .
At the end of the fusion I checked the spectral signatures and what is surprising that the spectral signature of the water has changed in the two new merged images (Landsat8 with S1) and (S2 with S1)

At the end of the fusion I checked the spectral signatures and what is surprising that the spectral signature of the water has changed in the two new merged images (Landsat8 with S1) and (S2 with S1).

Thank you for your help