Not believing all newer readers wouldn't render the greys correctly, I looked at mine again with some adjustments. At the default settings, the greys appear as you said: 1 and 2 appear white; 2 to 5 are light grey and all appear to be the same; and 6 and 7 are darker than 2 to 5. I originally had my settings a bit darker, so they all appeared different on my reader. Even though when set to default where they look like only 2 shades of grey, the screenshot still turned out showing all 7 shades as I posted above. And that I really find odd.
Perhaps a better test might be to use one of the images from this post.
I used the 2nd image from the first post and stuck it into the epub and tried again. This time, all 16 shades were visible on even the default or lightest settings for my reader.
Maybe it's just a case of the original greys not having enough variance between them, so they don't include the 16 shades of grey that e-ink will display, if I'm making sense how I'm trying to describe it.