The browser might have something to do with it.
If you really want to know what you’re looking at, you should make your own step wedge image of varying degrees of black, and view that in Photoshop on your display. Then you can eliminate the browser from the equation.
If you’re looking at the Dry Creek web page I’m thinking of, then this is a very discriminating test and you would expect to be able to see the first patches except on the very best of equipment in the very best environment.