The image is essentially a blurry two tone image, black and white. I went from the colour image, to gray scale, then made the curve very steep...so it was essentially black and white, then blurred it. It's the blurred areas where this phenomenon is occurring, no curves have been applied after the blur.
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I'm not clear on why you chose this particular way of printing a grayscale image. On a new image file with white background, I make a rectangular selection, say 6x1 inches horizontally. I reset the color picker to black & white, and then use the gradient tool set to Linear, drawing a line with the shift key down so that the gradient will go from one end of the selection to the other. I get a smooth gradient from black to white.
I then make a second selection, equal in size to the first, directly under it. This time I drage the gradient from the right side to the left, just to get the black & white reversed.
I then posterize the second gradient into an odd number of "colors", with no dithering. This gives me a series of 3 steps, or up to 13, which I use for testing the neutrality and tonal range of the paper. The top gradient is smooth, and can show banding, while the bottom gradient consists of discrete steps of tone.. You could even duplicate the horizontal gradients and rotate them 90 degrees to see if banding or blurring shows up in that orientation, as well. I have never had the problem you describe on my Z3100.
So far, I have printed the tests in color, often embedded in a larger test image I use. I have been thrilled with the accuracy and neutrality of the Z3100 (I use Qimage to control the color, but you could use Photoshop, of course). The papers I have used so far have been HP's Premium Glossy, Epson Premium Luster, Epson Enhanced Matte, Kodak Professional Glossy, Bergger PN33 textured fine art paper, and DigiPros Matte Canvas.
The printer-made profiles differ in how deep the black is that can be printed, but not IN neutrality.