I have read that during the third Khalief, Oethmaan, a committee under supervision of Zaid ibn Thabit has been ordered to make an official editorship of the whole Koran. But this "Oethmanian" text didn't give yet a uniform reading. In the first half of the fourth/tenth century the 'imaam of the Koran readers' in Baghdad, Ibn Moedjahid, gave a solution to this problem. He said that the word harf should be put equivalent to kira'a. He declared seven ways of reading correct. Because according to him the saying of the Prophet (PBUH) that the Koran was send in seven ahroef means that seven ways of reading are inspired. Nowadays only to ways of reading are in use, Warsj of Nafi' and Hafs of 'Asim. Could you tell me more about these different ways of reading? Are there ahadieth about this?.
Firstly : You should note, may Allaah bless you, that the Quran was revealed in one style at the beginning, but the Messenger of Allah (peace and blessings of Allah be upon him) kept asking Jibreel until he taught him seven styles, all of which were complete. The evidence for that is the hadeeth of Ibn ‘Abbaas who narrated that the Prophet (peace and blessings of Allaah be upon him) said: “Jibreel taught me one style and I reviewed it until he taught me more, and I kept asking him for more and he gave me more until finally there were seven styles.”
(narrated by al-Bukhaari, 3047; Muslim, 819)
Secondly: What is meant by styles (ahruf, sing. harf)?
The best of the scholarly opinions concerning what is meant is that there are seven ways of reciting the Qur’aan, where the wording may differ but the meaning is the same; if there is a different meaning then it is by way of variations on a theme, not opposing and contradiction.
Thirdly: Some of the scholars said that what was meant by ahruf was the dialects of the Arabs, but this is far-fetched, because of the hadeeth of ‘Umar ibn al-Khattaab who said: “I heard Hishaam ibn Hakeem reciting Soorat al-Furqaan in a manner different from that in which I used to recite it and the way in which the Messenger of Allaah (peace and blessings of Allaah be upon him) taught me to recite it.  I was about to argue with him whilst he was praying, but I waited until he finished his prayer, and then I tied his garment around his neck and seized him by it and brought him to the Messenger of Allaah (peace and blessings of Allaah be upon him) and said, ‘O Messenger of Allaah, I heard this man reciting Soorat-al-Furqaan in a way different to the way you taught it to me.’ The Messenger of Allaah (peace and blessings of Allaah be upon him) said to him, ‘Recite it,’ and he recited it as I had heard him recite it. The Messenger of Allaah (peace and blessings of Allaah be upon him) said, ‘It was revealed like this.’ Then he said to me, ‘Recite it,’ so I recited it and he said, ‘It was revealed like this.’ This Qur'aan has been revealed in seven different ways, so recite it in the way that is easiest for you.’”
(Narrated by al-Bukhaari, 2287; Muslim, 818)
It is known that Hishaam was Asadi Qurashi (i.e., from the clan of Bani Asad in Quraysh) and ‘Umar was ‘Adawi Qurashi (i.e., from the clan of Bani ‘Adiyy in Quraysh). Both of them were from Quraysh and Quraysh had only one dialect. If the difference in ahruf (styles) had been a difference in dialects, why would two men of Quraysh have been different?
The scholars mentioned nearly forty different opinions concerning this matter! Perhaps the most correct is that which we have mentioned above. And Allaah knows best.
Fourthly: It seems that the seven styles were revealed with different wordings, as indicated by the hadeeth of ‘Umar, because ‘Umar’s objection was to the style, not the meaning. The differences between these styles are not the matter of contradiction and opposition, rather they are synonymous, as Ibn Mas’ood said: “It is like one of you saying halumma, aqbil or ta’aal (all different ways of saying ‘Come here’).”
Fifthly: With regard to the seven recitations (al-qiraa’aat al-saba’), this number is not based on the Qur’aan and Sunnah, rather it is the ijtihaad of Ibn Mujaahid (may Allaah have mercy on him). People thought that al-ahruf al-saba’ (the seven styles) were al-qiraa’aat al-saba’ (the seven recitations) because they happened to be the same number. But this number may have come about coincidentally, or it may have been done deliberately by Ibn Mujaahid to match what was narrated about the number of styles (ahruf) being seven. Some people thought that the styles (ahruf) were the recitations, but this is a mistake. No such comment is known among the scholars. The seven recitations are one of the seven styles, and this is the style that ‘Uthmaan chose for all the Muslims. 
Sixthly: When ‘Uthmaan made copies of the Qur’aan, he did so according to one style (harf), but he omitted the dots and vowel points so that some other styles could also be accommodated.  So the Mus-haf that was copied in his time could be read according to other styles, and whatever styles were accommodated by the Mus-haf of ‘Uthmaan remained in use, and the styles that could not be accommodated fell into disuse. The people had started to criticize one another for reciting differently, so ‘Uthmaan united them by giving them one style of the Qur’aan.
Seventhly: Your saying that Mujaahid’s different recitations meant the seven styles (ahruf) is not correct, as was said by Shaykh al-Islam ibn Taymiyyah.