Short description of error
When using Adaptive sampling render option (Cycles+Branched path tracing) down sampling happen on volume scattered shaded objects.
If one applies volume scatter shader for an object, like for generating cloud,
and this cloud is in front of an environment texture where adaptive sampling is applying less samples,
lesser sampling is applied to the cloud itself.
This is always happening to volume scatter shaded objects. Then, sample rate applied is same as for background object, but, correctly, it should be max samples.
Adaptive ON has more artifacts. Best to see difference is to compare cloud edges.
Exact steps for others to reproduce the error
Blend example... (Render frame using Adaptive ON and OFF.)