Quotations are copious in both written and oral texts. Formulae are also a sort of "quotation", although they lack (the memory of) an author. Moreover, until the advent of print, in many places "the physical act which produced originals was the same as that which produced copies. Writers were responsible for both" (Burrow 1982: 31). Hence, they were very likely to be often used to both copying and actively writing, and to use the first ability within the second.
Burrow quotes a famous passage of St. Bonaventure (XIII c.) about the modus faciendi librum (way of making a book):
There are four ways of making a book. Sometimes a man writes others' words, adding nothing and changing nothing, and he is simply called a scribe [scriptor]. Sometimes a man writes others' words, putting together passages which are not his own; and he is called a compiler [compilator]. Sometimes a man writes both others' words and his own, but with the others' words in prime place and his own added only for purposes of clarification; and he is called not an author but a commentator [commentator]. Sometimes a man writes both his own words and others', but with his own in prime place and others' added only for purposes of confirmation; and he should be called an author [auctor] (St. Bonaventure, fourth quaestio in the Proem to his commentary on Peter Lombard's Sentences, quoted in Burrow 1982: 30-31).
So, no one just writes without any model: "The scheme simply does not allow for that possibility" (Burrow 1982:31). Moreover, how reliable is Bonaventure's description of the scriptor? In fact, scribes did not "change nothing". On the contrary, the scribe added and changed "not only inadvertently, like the compositor, but also deliberately. He replaces obscure expressions with more familiar ones, omits and rewrites passages, and sometimes adds passages from other sources or even passages of his own composition" (Burrow 1982: 31-32). At least in India, the case I am more acquainted with, a copyist may act so in order to alter the text, but, more often, in order to improve it or to better preserve it (hence, explanations may be added whenever a passage seems to obscure to be understood). Hence, s/he does not feel like someone who is violating the text, but rather, as its protector. Was Bonaventure unaware of that? My personal answer is that he did not list such editorial changes as "changes". The only changes worth mentioning were the ones made by a compiler, all others were just part of the process of copying. Nowadays, we are also used to the idea that a publishing house' editors are allowed to practically re-write a proposed text, and yet they are not mentioned as co-authors.