The Holy Qur'ān itself testifies that it was not revealed in a single instance and that it came down piecemeal.
وَقَالَ الَّذِينَ كَفَرُوا لَوْلَا نُزِّلَ عَلَيْهِ الْقُرْآنُ جُمْلَةً وَاحِدَةً كَذَلِكَ لِنُثَبِّتَ بِهِ فُؤَادَكَ وَرَتَّلْنَاهُ تَرْتِيلًا
The rejecters ask, "Why is the Qur'ān not revealed to him in a single instance?" We have revealed it this way so that We fortify your heart through it. We have recited it to you gradually. (25:32)
Parts of the Holy Qur'ān were revealed considering the circumstances of its addressees. Therefore, when some of the directives were relaxed in some aspect or some others were complemented by new revelations, the new addition was put next to the original command. Sometimes these later revelations were placed at the end of the discussions they complemented. Since the shifts in the discourse have been marked by the rukū' division, people find it difficult to understand the interrelation of such complementary passages with the preceding discussions. We can study this interrelation between the two by the help of the following facts.
God Himself has explained that such verses are explanatory verses, revealed in accordance with His promise to the Prophet (sws) that He would explain whatever called for an explanation and would arrange it in a set order where He says:
فَإِذَا قَرَأْنَاهُ فَاتَّبِعْ قُرْآنَهُ ثُمَّ إِنَّ عَلَيْنَا بَيَانَهُ
When We have recited it to you follow that recitation of Ours and then We shall Ourself explain it. (75:8-9)
The complementary verses have been expressed in a different style. They do not follow the pattern of the verses that follow and precede them. This also serves as an indication to the fact that they are explanatory verses.
Some of the explanatory verses betray by their style that they respond to an implicit question. Many such verses clearly indicate that they are pointing out some matter undercurrent in the discourse.
It needs to be appreciated that some sūrahs have been put on the tongue of the Prophet (sws) and some on the tongue of the Gabriel and many more on the tongue of the Almighty himself, following the style of the earlier revelations. The Holy Qur'ān has clearly explained this fact:
وَمَا كَانَ لِبَشَرٍ أَن يُكَلِّمَهُ اللَّهُ إِلَّا وَحْيًا أَوْ مِن وَرَاء حِجَابٍ أَوْ يُرْسِلَ رَسُولًا فَيُوحِيَ بِإِذْنِهِ مَا يَشَاء
It is not possible for any mortal man that God should speak to him (face to face) except (that he does so) by revelation, or from behind a veil, or though a messenger sent and authorized by Him to make known His will. (42:51)
At another place God Almighty says:
قُلْ مَن كَانَ عَدُوًّا لِّجِبْرِيلَ فَإِنَّهُ نَزَّلَهُ عَلَى قَلْبِكَ بِإِذْنِ اللّهِ
Say, "Whoever is an enemy to Gabriel (must know) that he [Gabriel] has revealed it to your heart by God's command." (2:97)
Yet another place the same has been put thus:
إِنَّهُ لَقَوْلُ رَسُولٍ كَرِيمٍ ذِي قُوَّةٍ عِندَ ذِي الْعَرْشِ مَكِينٍ مُطَاعٍ ثَمَّ أَمِينٍ وَمَا صَاحِبُكُم بِمَجْنُونٍ وَلَقَدْ رَآهُ بِالْأُفُقِ الْمُبِينِ وَمَا هُوَ عَلَى الْغَيْبِ بِضَنِينٍ وَمَا هُوَ بِقَوْلِ شَيْطَانٍ رَجِيمٍ
This is a word of a gracious and mighty messenger, held in honor by the Lord of the Throne, obeyed in heavens, faithful to his trust. No, your compatriot is not mad. He saw him on the clear horizon. He does not grudge the secrets of the unseen; nor is it the utterance of an accursed devil. (81:19-25)
Still at another place the Almighty says:
إِنَّهُ لَقَوْلُ رَسُولٍ كَرِيمٍ وَمَا هُوَ بِقَوْلِ شَاعِرٍ قَلِيلًا مَا تُؤْمِنُونَ وَلَا بِقَوْلِ كَاهِنٍ قَلِيلًا مَا تَذَكَّرُونَتَنزِيلٌ مِّن رَّبِّ الْعَالَمِينَ
This is indeed the utterance of a noble messenger. It is no poet's speech: you seldom believe! It is no soothsayer's divinations: how little you reflect! It is a revelation from the Lord of the universe. (69:40-3)
This issue has been extensively discussed in my book 'Asālīb al-Qur'ān (Styles of the Qur'ān).
The above discussion, it is hoped, has made it clear that the Holy Qur'ān contains verses which have been incorporated later as required explanations or as complements to some earlier discussions. Many verses have been put in the mouth of Gabriel. Many others issue from the Prophet (sws). A major portion however is the direct speech of God. In order to understand the Holy Qur'ān and interpret it you have to make out which verse issues from whom and who it is addressing. Without successfully identifying the addressors and the addressees you will not be able to understand the coherence in the text. This can be explained through an example. In an allegorical story many characters are made to talk according to their role in the plot. If someone assumes that all these things have been told by a single character he will certainly fail to make out any connection between the parts of the whole story.
This example I have offered only in order to explain my point fully. The word of God, indeed, does not suffer from such limitations. His word is exalted and does not stand in need for such examples and analogies.