According to Sunni Muslims, copies of the Koran mirror an eternal heavenly exemplar. However, early accounts of how the Koran was compiled tell a very different story. For instance:
Zaid later said, "I then searched out for the various parts of the Qur'an, finding them preserved on palm branches, on the surfaces of flat stones, in the hearts of men, on pieces of leather, and on (the) shoulder-bones (of camels and/or sheep). The Biography of Abu Bakr As-Siddeeq, 519.
You can read similar accounts. Let's assume for the sake of argument that the angel Gabriel spoke to Muhammad. Problem is, the text of the extant Koran is a compilation of oral traditions. Recollections of what Muhammad said. So that creates a gap between what Gabriel allegedly said to Muhammad, and the actual record.
i) There's no way to sift apocryphal sayings of Muhammad from authentic sayings.
ii) At best, people generally remember the gist of what someone said, not the verbatim statement.
Although the Koran contains autobiographical material, it's an oral history, passed down by word-of-mouth. Sayings attributed to Muhammad. But there's no way to authenticate the sayings. And even if apocryphal sayings could be eliminated, the remainder paraphrase what people remember that he said. They don't preserve the exact wording.