Sounds like your common variety type backlight bleed, maybe a little more pronounced than usual. Many LCDs have it, some worse than others.
It can sometimes get better, and there are some methods online you can try to make the screen more uniform, but I wouldn't worry about it if I were you. There's nothing wrong with your DSi XL is that's what you were thinking. Just try not to pay attention to it and you'll forget it's even there.








