That’s odd. That’s something I’d actually consider a bug 
Overall, combining RTAO and SSGI will likely still give you bad performance and double occlusion. It really depends on the scene, but people often tend to overestimate the amount of occlusion scene should get.
Then again, maybe it looks closer to reality, as SSGI alone won’t do it for dynamically lit scenes, because there is no large skylight occlusion to begin with. So double occlusion, albeit wrong, looks more right because it somewhat simulates something close to skylight illumination. In that case, disabled RTAO but enabled Ray Traced Shadows on Skylight combined with SSGI would probably be the closest to the right result one can get. But then, once again, RT SkyLight is much more expensive than RTAO
Hard choices…