Sentinel 2 cut area

I want to cut interest area. I made ndvi image.
I cut band image in qgis and calculate ndvi. But ndvi rate was wrong.
could I cut area in snap? If I cut interest area, I want to get histogram with csv file.
I need your help.

You can load vectors into SNAP or digitize polygons in SNAP and use them as area of interest in the histogram tool. An explanation is given in this, chapter “Working with vectors”, page 21-22: Synergetic use of S1 (SAR) and S2 (optical) data and use of analysis tools

A possible reason why the values of the exported raster are different from those in SNAP is given here: Export of products from SNAP