That is only one of many things in Unreal that are not quite perfect. I take it you only got this game recently and you did not jet tried any of the cheat-codes. I´ll give you one (try find the rest by yourself). Press the tab key and type: "ghost"+enter (type "walk" to do the opposite). Now you can move fly freely through the levels. You´ll notice again that from certain angels you can´t see certain things, like a piece of wall, water, or another something else.
Still, the question remains, why does this happen?
It all comes down to the entire constuction of an unreal level. Making them is a difficult thing, but when you get the hang of it, the results can be quite spectacular. And the more complex things get, the slower they will run on your computer. To make sure that an unreal level will run on an 'average' computer system like a P2 300, you would have to combine both creativity with simplicity. So a sphere in unreal is not actually a perfect spere but something that looks more like a football, constructed out of triangles and pentagons. But even when you try to keep things simple, it could run slow on your computer. To cope with this annoying problem, the creators of unreal came with some simple but efficiët solutions. For example (I´ll just keep this simple), when you are in a large room, the room behind the door would normally be drawn (in 3d that is) and computed as well as the room you are already in. Of course, that isn´t really necessary. Why draw something you can´t see? Unreal works with zones. That means that when you are in a certain place, only that what you can see is beïng drawn and calculated. And when are somewhere else, the computer will only calculate that part of the level. So a wall on the edge of a level will only have to be visible from one side, because you´ll never even be on the other side of the wall to see it. That kind of explains the whole thing. Obviously the creator of "SkyCaves" made a small error so that a player can see something from a certain angle he isn´t suppost to see (or he cannot see something he is suppost to see /~unreal/ubb/html/wink.gif.
I hope I answered your question, it took me five minutes of my life to write it down.
:¬)