From the tests I runned until now for 5 points:
1. Always the first row of the viewshed tool is the non visible area.
2. If you run the viewshed tool for each of the 5 points separately you will have different visible and non visible areas than if you run the shp with the 5 points at once in the viewshed, however the non visible area remain always the same, the first row with value "0".
3, If you run the process from the observer points you will get the pixels that are visible from each point "the raster that can be seen only by the specific observer" which fits with the viewshed if you run it separately for one point (1.) each time, but it has very big differences with the viewshed that comes up if you run the viewshed tool with input the shp where all points are together in one file (2.)
! So the non visible area is always the first row even if you run the viewshed for 1 point or for 15.
! Still remains unknown why the viewshed raster has less rows than the number of points you used.