The "Radius" parameter of "GI surface details" only affects detail that's too small to be captured in the GI cache, because the cache has to be interpolated. Overall, GI is still calculated globally.
In any perspective render we might have objects which are very close to the camera and others that are very far away. The renderer needs to consider certain things in screen space (perspective) to deal with this huge range of sizes and distances. If GI surface details were calculated in world space then it wouldn't be possible to add detail to a distant mountain efficiently at the same time as adding detail to a foreground rock. The GI cache is calculated with dots spaced evenly in screen space. Similarly, GI surface details adds in missing details on a scale that is calculated in screen space. To generate a realistic image, the GI cache and the GI surface details should work together to produce a reasonable GI solution. If it's working well, you should not notice much change as the camera moves, but some change is inevitable because it is only an approximation.
Matt
