Edge trimming doesn't cut off the geometry - it looks at where the pivot point is, and if it is inside the edge boundary, the instance is rendered - even if the geometry then "sticks out" beyond the edge. This is to stop things like flowers cut in half, rocks cut in half, etc. It is possible to do such a hard cut, you could use a Corona slicer for instance, or Corona Distance Maps into opacity, but it's not the aim of the Edge Trimming.