The lack of understanding is largely due to poor explanation in the RFM. In the types on which I teach, you are supposed to droop Nr to continu the T/O "to min 90%"
The point is - drooping the Nr is the only way that you are extracting the full power from the remaining engine. If its not drooping, there is something left which you are not using.
The figure of 90% of course is a LIMIT, but lots of people, including many instructors see it as a TARGET