They are there because the limitations of a 8bit/channel png and also the limitations of our monitors. If you check the RGB values, you will see that all three values will decrease only by 1 for each band, and there's only 256 step of gray.
Practically there's no room for finer gradients in the 24 bit color space (8 bit / channel). You can't even blur them after rendering it.
You could try setting it to 16bit/channel png and on the Output panel in the render settings, and turn up dithering in the post processing panel. It will help a bit, but not much.
I don't know if Pioneer supports 16bit/channel png's. The file size will be at least double, which might be a problem.