Trying to use commands failure

Wen I use the "fill" command and set in the coordinates it has problems. Eg my cordinates -50 120 -50 my fill comand ~-50 ~120 ~-50 ~50 ~120 ~50 so it is a small surface and in my surrounding area. But then it either places it a 100 blocks to high and 50 blocks to the one side. Or it says it is outside the world boundaries. The vidio added indicates where it fills at the wrong coordinates.