Why is there a difference between NDVI using Sentinel 2, in ArcMap & QGIS?

Arcmap is notorious for retaining the raster type and not treating values as decimals/float when calculated from 16bit integer rasters.

In order to force Arcmap to treat raster calculation result as a float, you should multiply one of the arguments by 1.0, or simply append a '.0' to a any constant or real number in the formula.

In your case (NIR-Red)*1.0/(NIR+Red) will suffice.