Ah, that explains the black bar they used along the bottom. I never noticed this until I turned it clear so I could compare it to yours. To my eyes, it almost makes it look like an old book with a slightly blackened bottom edge. I kind of like it
The Paperwhite has the same issue, but I found it very distracting (it's a lot more pronounced, looks like bleeding ink more than a shadow). Clever of Kobo to hide it with the default UI setting, although it really isn't that bad.