This is the effect of the 8x8 blocks into which the image is divided for compression added to quantization noise The color at each side of the boundary of two blocks is not the same, it is almost the same producing vertical bands. You don't see horizontal bands because in this image there is no color variation in the vertical direction.

Another problem with jpeg is for selecting or extracting a part of an image by color boundary. An image of an object on a black or other color background, once saved the image as jpg, the black is not more RGB 0,0,0. It is black with random pixels with a dark color. Using black for selection you pick the objects with noise pixels and many times with not clear defined borders and then you have to clean the mess. If you saved the image as png, bmp, tiff, you never have this problem.

Stupidity also evolves!