There are hundreds of examples in the Synoptic Gospels where two authors have the same story in slightly different words. To be sure, they tend to agree more closely when quoting Jesus than they do when narrating the background of the story, but the pattern is still quite evident.
William Farmer once described the patterns of agreement & disagreement in wording (and order) as the "fundamental problem" in the Synoptic Problem (which studies the relationship among the texts of Matthew, Mark, and Luke).
I'll address the subject in 3 parts:
- What did Jesus say
- Why did Gospel authors vary their words
What did Jesus say
Jesus was an itinerant preacher--He would have said a lot. Yet every word attributed to Jesus in the Gospels can be read aloud in under an hour. Clearly, there is much that He said that we don't have, and there is every reason to believe He would have told the same story/taught the same principle more than once. A good teacher can tell the same story many times without using exactly the same words each time.
Furthermore, although my research leads me to the conclusion that Jesus would have been able to speak Greek, in the custom & culture of the time Aramaic & Hebrew would have been the more readily used languages in religious teaching. (see my video "What languages did Jesus speak")
Since the Gospels have come down to us in Greek, what we have (at least in most cases) is a translation of what He said. There is no single correct way to translate an idea from one language to another, so variations in the wording used (in translation) is not surprising--it should be expected.
Why did the Gospel authors vary their words
Virtually all Synoptic scholars in the last 300 years have agreed that there is a literary relationship between the Gospels of Matthew & Mark, although there has been much debate on the sequence & process of their composition.
Allowing that whoever wrote second used the other author's work as a source or guide, the reason for the variation in wording is fairly straightforward.
It takes two hands to manipulate a scroll, and writing desks as we know them today did not exist in the first century (people usually wrote with the scroll on their laps). As such, it would not be physically possible for someone to write on one scroll while simultaneously reading from another. Their options would have been:
- Lay the scrolls out next to each other and crawl back and forth between the two
- Read one document, then later write a new document and quote from memory
- Have a second person assist in the process--one reads from the source scroll and the other writes on the destination scroll.
In all cases, the words pass through human memory (either short-term or long-term memory, depending on the option above that is used), and as a result, in the Synoptic Gospels we get numerous nearly word-for-word agreements with slight variation.
If you're interested in a technical discussion of how this worked (whether copying or translating a document), I recently posted a video on my channel: The Synoptic Problem - Patterns of Agreement & Disagreement in Wording
The OP asked about chronology--it's perfectly true that none of the Synoptic authors are trying to present their material in a strictly chronological fashion (Luke says he's providing an orderly account in Luke 1:3--we read that with Western eyes and assume he means chronological order, but he never says that).
Matthew's Gospel is largely organized by topic. Luke's is largely organized by geography. The answer for Mark depends on your solution to the Synoptic Problem. Mine suggests Mark wrote 3rd and alternated following the order of the Gospels that preceded him, Matthew & Luke.
Disclaimer - the sources cited above are my own research on the Synoptic Problem