It seems that there are some PNGs that are stored in several chunks (or that's what I believe), because XnView only displays half of them (and by half I mean half of the image). Depending on the image size (width, height), XnView can display the complete image, the top vertical half, the top vertical third, etc.
Both are exactly the same image, one is twice the size of the other. On the first one (x2) you can see only half of it. On the second one, you can see only 1/4 of it.
Thanks in advance and regards,