Here is an explanation given by Sayyid Sa'eed Akhtar Rizvi in his book, The Quran: Its Protection from Alteration.
The commentator of al-Wafiyah, Muhaqqiq al-Baghdadi has clearly stated, by quoting from Muhaqqiq al-Karaki (who has written a complete tract on the subject) that: "The traditions which speak of omission must either be reinterpreted or rejected. Any tradition which is contradictory to the Qur'an, the acknowledged sunnah and the ijma' must be discarded if it has no room for interpretation or justifiable explanation."
A tradition, recorded in al-Kafi is quoted here to give an example in practice of what we mean when we speak of reinterpretation or justifiable explanation: Abu 'Abdillah [al-Sadiq] (peace be upon him) said, "The Qur'an which was brought by Jibrail (peace be upon him) to Muhammad (Mercy of Allah be on him and his progeny) is seventeen thousand verses."
Shaykh as-Saduq has written in his Kitabu'l-Itiqaddat, what in my view amounts to a reinterpretation of this hadith. He writes, "We say that so much of revelation has come down, which is not a part of the Qur'an, that if it were to be collected, its extent would undoubtedly be 17,000 verses. And this, for example, is like the saying of Gabriel (a.s.) to the Prophet (peace be upon him & his progeny): 'Allah says to thee, O Muhammad, deal gently with My creatures, in the same manner as I do."'
He goes on quoting many such ahadith qudsiyah until he concludes by saying: "There are many such (ahadith qudsiyah) all of which are revelations, but do not form part of the Qur'an. If they had been (part of the Qur'an), they would surely have been included in it, and not excluded from it."
If one is not prepared to accept this explanation because the tradition speaks about "the Qur'an", then we will discard this hadith without hesitation. Although the number (17,000) given in this tradition is much smaller than the one given by the 2nd Caliph (one million and twenty seven thousand letters in the Qur'an), it is never-the-less three times bigger than the actual number of the verses (which is some six thousand two hundred and thirty six verses).
This is the Shi'a method of resolving the problem of the ahadith on tahrif. Anyone studying it with unbiased mind and heart would readily accept it as the only correct solution for such ahadith because it is based on the method formulated by the Prophet (peace be upon him & his progeny) and expressed by Imam Ja'far as-Sadiq (a.s.) as follows: "...And so whatever conforms to the Book of Allah, take it; and whatever is contrary to the Book of Allah, leave it .. "
Source - https://al-islam.org/quran-its-protection-alteration-sayyid-saeed-akhtar...
