They don't look like bitrate issues to me in the images, more likely a result of chroma smoothing or similar processing to reduce noise. Going on the amount of detail present in those image, I highly, highly doubt that increasing the bitrate to go dual layer would make a single pixel's difference.

Also, remember these are jpeg images that were posted, so they have been through a blocking compression method - you would need to see them posted here in a different format to draw any conclusions.