Similar to how they detect spam by comparing similar emails (eg, same sender, largely the same content, etc). So the first maybe 1,000 or so see the spam in their inbox, but after enough people report it, everybody else gets it in their spam folder.

So they could learn that for all these emails with largely the same content, this one image has a slightly different URL, but the image is always the same (or similar). So as with spam, the marketers might see the first few "opens", but once Google learns that they all similar anyway, the won't see any more.

Now, I don't know if that's what they are doing, but its certainly possible.