Ibn Sa'd was much more than just a mere copyist.
Nitzsch, however, held that this was a copyist's gloss, harmonizing with the received Boetius legend, which had been transferred to the text, and did not consider that it outweighed the opposing internal evidence from De Cons.
Hagenmeyer inclines to believe in an original author, distinct from Albert the copyist; and he thinks that this original author (whether or no he was present during the Crusade) used the Gesta and also Fulcher, though he had probably also "eigene Notizen and Aufzeichnungen."
Let us suppose that from a text which we will call A a copy has been made which we will call B, and from this again a copy which we will call C. If the copyist of B goes wrong once and the copyist of C twice in a hundred times, then, assuming that there is no coincidence or cancelling of errors, the relative correctness of the three texts A, B, C will be zoo (absolute correctness), 99 and 97.
Then the copyist's eye is apt to slip from the first of two similarly written groups to the second; and he will thus omit all that is between.