If you are interested, the Q12 image is also available . Tom Lane notes that the point of JPG is to reproduce photographs that appear as accurate to the eye as possible with minimum file size. Perhaps it is unfair to blow up an image to the point where the pixels are visible. The above discussion should be a warning to us when we talk about the quality of JPG compression. JPG images differ not only in the quality setting of the compression program, but also in the algorithms used by the program authors in achieving that compression.
Some programs make different decisions in how to compress.
One should note that Adobe's approach may not be the wisest choice for compromising between file size and image quality. The eye is less sensitive to high spatial frequencies than to high frequencies in intensity. This means that a good JPG algorithm should store chroma information with less detail for optimum balance between file size and quality. In very special images such as this one, the problem is obvious even unmagnified. Each approach offers advantages in some circumstances.