NIS Upscaling

I have been trying the implemented NIS upscaler for a little now and i am wondering why it looks much worse than the NIS upscaler from the steam software "lossless scaling".

When compared at a native resolution of 4k and a scale factor of 1.5 (render resolution WQHD), the lossless scaling version looks significantly better than the implemented version at equivalent scale (50%).

Can someone explain why thats the case? I would have assumed them to look pretty much identical but thats not at all the case.

I've tried it as well and it seems NIS in client is used only as a sharpener. There is no true upscaling in a sense.