An Analysis of Lightroom JPEG Export Quality Settings

The difference from the first example is stunning, and relates to what visual changes human are sensitive to: we pick up on imperfections in a continuous tone much more readily than slight changes in varied detail. The JPEG compression algorithm is built around this difference, trying to preserve quality in these smooth gradient areas, but as well as it does, a photo like the sunset presents a daunting challenge.

That reed mat image is interesting...