I have two ideas:
- You checked the Normalize option
This changes the pixels of the rasters according to image statistics so that both contain the same range of values. Uncheck it if you need the absolute backscatter values. - The other explanation is that you maybe selected GeoTiff as an output format
I would recommend to stick to the Beam DIMAP format as long as possible. It is compatible with almost any GIS software and carries all metadata required for the processing of data in SNAP.
Conversion to GeoTiff causes loss of metadata and brings the risk of changed data type (eg. 16 bit unsigned integer can only carry positive values between 0 and 65,535). This often leads to changed pixel values.