I'd bet he's basing that on previous classes -- nothing personal, but he's probably found he grades harder at the outset than other professors or teachers his sutdents have had. Remember, most of his students have probably not been writing for years!

And, he just wants to prepare everyone & let them know it's a progressive grading system. I think that works for writing as you know, writers say "writing is editing."
Very interested in following your notes on this course!