We find that certain types of objects such as very dense, small scale scatters (rugs/carpets/gravel) can occasionally skew the noise threshold calculation for the render i.e. rendering the whole image might be at 10% after 20 passes, but region-ing around these objects can result in a "noise level" of 5% at the same number of passes, with the same visual quality across the image.
Allowing users to exclude certain objects from the calculation may help provide more accurate feedback on how many passes are needed for a render.