An image that seems to combine properties of bitmap/halftone and grayscale: Don't know how I did it.

I'm wondering if anyone can identify how the attached image (seen in two screenshots at different zoom leveles) might have been made. It's not making sense to me and I'm not sure how I got to it. But I'd like to be able to do it again.

Image 1 shows the image zoomed in to 600%.  We can see that the image (which is 180 ppi) is made up of small square units, and these units are in a range of values. While the image is RGB in this shot, to the best of my recollection, I first took it through a series of stages, from grayscale to bitmap to a halftone settting, back to grayscale, finally to RGB.

It makes no sense (I believe) that a bitmap/halftone image would be comprised of anything but dots of the same value. But this image, which I believe has been bitmapped/halftoned, seems to be made of dots in different values (The attached "Image 2", zoomed in much further, shows that this is definintely the case). 

I don't think this is just a basic grayscale or RGB image. I have a grayscale version of this image that didn't go through much processing. It looks much different when zoomed in to 600%. You can't make out any individual squares at 600% (If I go in a lot further, then yes I start to see individual pixels, but nothing like that at 600%). So to me it seems that these attached images reflect that the file went through some processing, which caused it to be broken up into discreet dots or units, and it doesn't simply function as a grayscale image. It has halftone dots in a range of values. I'm unable to reconstruct how I got to this, but I'd like to, if anyone has any ideas.