arcgis desktop - Why getting wrong NDVI values resulted from scaled reflectance in ArcMap?

I have made atmospheric corrections to my Landsat 8-OLI raster using 'ENVI FLAASH' having surface reflectance values scaled between 1-100.

In ArcGIS I am using simple expression to compute NDVI using 'Raster Calculator' tool as illustrated below.

However, the results are not respectable. The same results are faced using 'ENVI Band Math' tool. Interestingly, this is not the case with the outcome from 'Spectral Indices' or 'NDVI' tools built within ENVI.

Why there is great discrepancy in results in all these cases?

Answer

you should either set the calculation in float 'Float(("nir"-"red"))/("nir"+"red")' or multiply by 100 '(100*("nir"-"red"))/("nir"+"red")'. Your strange results are due to conversion to integer (the default).