It appears that the ColorRamp (I'll try to spell Colour American) has no effect when paired with the Ambient Occlusion node. There is also a vast difference between the RPR preview render and Final Render with this particular node setup.
I have included a demo file and screenshots which demonstrate the issue(s).
The expected output should be more like how EEVEE displays the result where the ColorRamp has squeezed the AO towards the edges of the object.
If you add in and then remove the ColorRamp and do a final Render in RPR saving each one to a different render slot you can see that it is exactly the same output = therefore ColorRamp is having no effect at all.
Test was done on MBP 2017 with macOS latest everything using one eGPU = RX 580 with GL Interop = False.
RPR Final Render:
Node Tree for testing:
Sample file attached below.