Good catch - looks like it's a PNG image, with an alpha channel for the rounded corners, and a subtle gradient in the background. The gradient is rendered with dithering, to prevent colour banding. The dither pattern is random, which introduces lots of noise. Since noise can't be losslessly compressed, the PNG is an enormous 6.2 bits per pixel.
While working on a web-based graphics editor, I've noticed that users upload a lot of PNG assets with this problem. I've never tracked down the cause... is there a popular raster image editor which recently switched to dithered rendering of gradients?
My reasoning is because once upon a time, I was using Macromedia Fireworks, and PNGs gave far far better results than JPGs did at the time, at least in terms of output quality. Nearly certainly because I didn't understand JPG compression, but for web work in the mid 2000s PNGs became my favourite. Not to mention proper alpha channels!
...and so it's stuck, two decades on haha