Why does Reduce Motion make my lockscreen wallpaper not fit?

Hi guys,

So, I'm not entirely sure about the parallax effect on iOS7, at least on my current 4S, and I was interested how it would be without it. After switching it off, the first thing I notice is that my lockscreen wallpaper (a photo taken with this 4S under iOS6) now is much lower down the screen, i.e. there's a black bar along the top, as you can see in the screenshot.

https://www.dropbox.com/sc/z0zdjmltg6nn6nz/ZDWvJ5-56r

Anyone know what's causing this?