It's probably got something to do with the particular rendering engine being "unbiased", but I'm unsure myself.

I notice that if I give Metropolis Light Transport mode (TR2 in Thea) enough passes in Keryky, even the most tiny detail will be captured, but at the expense of time and the graininess factor. Also the number of polygons given to the smaller details will affect reflection detail as well.
More polys kill render time though.

I know some modes I've tried will make cleaner broad surfaces with little noise but have issues with tinier details.
Some render engines are better with rendering globally illuminated objects, and some modes are better with caustics and indoor light elements.