Here is a better explanation. I have scanned some USPS souvenir pages and some have half-tones for some images.
Before running through optimizer
After running through optimizer. I believe this is called a moiré effect
In some applications like scanner software have an option called descreen) to correct when scannin.
I had used Microsoft software included with Windows but found the SCF optimizer produced better images for same resulting file size during the resize process EXCEPT the moiré effect. It is easier than launch yet another program when the browser optimizer is easy to use and convenient.