Short sight occurs when light coming from distant objects is 'overfocused', so that the point of focus is in front of the retina. It occurs because either the eyeball is too long, or because the cornea is too curved. Despite maximum flattening of the lens, the eye is not able to focus the light rays further back and on to the retina.

Light coming from near objects requires a stronger focusing activity anyway, so in myopia light from near objects is more likely to be focused in the right place. 

People with short sight are not able to see distant objects clearly. Short sight or near sight mean exactly what the terms suggest. You are sighted (you can see), near (short) distance objects. Near objects (for example, when reading a book) can often be seen well. This is because when looking at near objects, the light rays come into the eye going slightly outwards. These will focus further back in the eye than light rays that come in straight from distant objects.

The diagram above shows the differences in focusing between a normal and a short-sighted (myopic) eye.