Lumen reflections are temporally upscaled(without TAA) in the reflection channel which is why it’s blurrier and more performant than your RT reference.
r.Lumen.Reflections.DownsampleFactor is put on 2 to save massive perf.
If you put it to 1 or 0, you will get full res reflections. But I imagine your and your customer won’t like the performance you’ll receive.
EDIT:
Another photo of the Lumen sphere from further away, can’t even see the cube in it’s surface
If the cube is small or far away then lumen with cull it to save trace performance.