[Benjaminsen]

While the JPEG encoder in Photoshop is likely a lot better than what's in your scanner, I am, similarly to others here, fairly convinced that the majority of the difference is in the sample rate by the scanner.

I did some vanilla JS that simulates scaling from high DPI vs sampling directly at a specific DPI and the result does resemble the result of a low resolution scan.

http://chrisbenjaminsen.com/shared/samplerate/

I am aware optics does not work exactly work like this, but it's a reasonable approximation.