When a nature scene is put together by an artist, it will be done differently than it is in nature. The artist will arrange things as they thing is most artistic, or most realistic, or whatever, but since it is unlikely that you will know all the rules of the universe, you are likely to end up violating a number of them without realizing it. You might scatter leaves about the forest floor, but in reality with wind working on them they're more likely to stay in low spots and blow clear of high spots, or all bunch up on the east side of objects. Sandbars will often form on the inside of a curve in a stream, and the streambank will get carved away on the outside of a curve. Tree branches will tend to not cross becuase they're growing towards the light, which is usually blocked by all the other branches in the tree. Perpetually shady spots will have less water evaporate, and will thus tend to support different types of plantlife than sunny spots nearby. There are a zillion things like this that we don't know or don't think of when we put together a scene, but (as was mentioned above), still might stand out as looking out of the ordinary becuase we've seen the ordinary a lot before. If we wanted to get everything right, we couldn't stop at just creating objects that exactly match reality in appearance; we would need an entire exact physics simulation of the universe to get it right, and an incredibly complex simulator even to handle a scene a few feet across that handled light distribution affecting plant growth, temperature, wind, aging of various materials over time, water erosion, effects of browsing by various animals, etc.; a nearly impossible task at the moment. We will have to make do by just sticking to a fraction of the more obvious ones, although one can refine ones work over time as things are noticed. (and of course one can simply ignore the rules anyway, since it is art after all.)