The speech or writing of a person, however skillful or eloquent he may be, will not be uniform in all conditions and circumstances. In particular, the works which appear in the days of any author's first writings are very different from the works which come after many years and as the result of much experience and repeated practice; the later works are almost always better. But the Qur'an, in that it was sent down in the course of 23 years, and in that it was revealed in various circumstances and like a long flowing river passed over various stoney places, rapids, narrows, valleys and plains, and witnessed many amazing events, it remained forever, like a spring, clear and fresh. The unity and harmony of the themes and the style and expression of the Qur'an are a source for wonder.
We see this wonder reach its apex, and notice in the contents of the Qur'an that many different subjects are dealt with, but the style and unity of expression remain constant. It is clear that if someone, when he reached a stage of mastery over a special subject, shows what he can do, he may display brilliance, but if he undertakes something in another branch of which he is not a master, he will not produce any distinctive work. But the Qur'an shines to an amazing degree in every area.
source : http://www.maaref-foundation.com