This is probably a duplicate, but it's very hard to search for something like that, so sorry in advance.
The distance at which objects can be seen depends on which angle they have on the screen. The view distance of objects that are directly in front of you is way shorter than in the left and right edge of the screen. So to see far away objects you have to look slightly past them.
I consider this a bug, because ...
1. ... it's not realistic, you can see thing that you look straight at way better than things in the corner of your eye.
2. ... when I set my render distance to e.g. 16, why shouldn't I be able to see the same 16x16 square that I can see when I fly way up and look down?
I think it's an issue with the distance fog. It's way more "dense" in the center of the view where it should be the opposite.
(In my opinion the fog could get removed completely, but that belongs to the Minecraft suggestions.)