Art & Academe gives:
Teacher's Com + Scribe's Scribe/2 + 1 + Teacher's Virtues
for Lecture Commentaries, and also comments that "Lecture commentaries will never be as high a quality as a tractatus written by the schoolman."
However if you assume that such a book is subject to the professional Scribe/Illuminator/Binder bonuses; and that the same professional scribe did the original writing then you're looking at a minimum of:
Teacher's Com + 3 + 1 + Virtues + 1 + 1 + 1 = Com + 7 + Virtues
As opposed to:
Com + 3* + 1 + 1 + 1 + Virtues = Com + 6 + Virtues * noting the errata for Covenants
Meaning that the Lecture Commentary is better than the Tractatus.
On the other hand if we assume that if we're using the Extended rules that the Commentary total includes the +3 for professionals then that's a lot of work in total for a sub-par Tractatus at Com + 4 + Virtues.
How do we square this circle?