The scholars have pinpointed many major causes for different narrations or wordings of what is apparently one hadith or one incident. The following are some of the major causes:
( 1) The incident narrated may only seem like one event or statement while, in reality, each narrator or narration is describing a different event or statement of the Messenger of Allah (peace be upon him).
(2) Sometimes the hadith is narrated according to its meaning only and the exact wording has not been preserved. Other narrators may narrate the same hadith while preserving the exact wording
(3) Narrators have different retentive strengths. Particularly if a hadith or anecdote is long, it is natural for a narrator to recall or to narrate more of an incident than others. Hence, one’s version of the hadith may be longer than that of another narrator.
(4) Narrators sometimes intentionally abridge the text of the hadith. This is considered permissible as long as the meaning is not distorted. Hence, one narrator may narrate a complete statement of the Prophet (peace be upon him) while another may only narrate what he feels is relevant at the time he is speaking.
(5) Sometimes a narrator is present for only a portion of a statement or discussion while another person may have heard the entire statement or discussion. Hence, when the two narrate the same incident, the narrations may be quite different, especially in length.
(6) The Prophet (peace be upon him) would sometimes put questions to the Companions and their responses in one setting would differ. When a narrator relates that particular event, he may relate the response that he heard while others may relate the response that they heard.
(7) Narrators do commit mistakes, even very proficient narrators. This is an obvious reason for why one narration may differ from another. The scholars of hadith spent a great deal of time and effort weeding out such mistakes.
(8) Finally, there is the problem of intentional fabrication and distortion. This obviously did not occur from the honest and trustworthy narrators. In any case, though, its existence did lead to differing narrations of the same hadith.
Taken from Commentary on the forty Hadith of An-Nawawi vol-1 page 156-157 (or computer counting 175-176)