After looking at a couple dozen pages in close-up, my impression is
that the Retoucher went through most of the manuscript, carefully but
rather mechanically retracing many drawing strokes and letters -- on
some pages, almost all of the text.
In some cases it seems that he was not quite sure of what letter he
was retracing. In some cases he may have misunderstood the drawing.
For instance, check the leftmost nymph at the top of page f73v
(Sagittarius). The Retoucher apparently mistook the bottom edge of the
nymph's leg for the top edge of a "barrel", and provided the rest
of the barrel -- presumably thinking that it had faded out completely.
Ditto for the next nyph. Presumably he then noticed the mistake
and so left the barrels unfinished.