Leveling kind of breaks down here. When you compare the capabilities of a lvl 1 to a lvl 10 its hard to think of them as the same species.

But lets assume that D&D fluff comes from found documents. The books were written in the D&D world by authors who lived in the D&D world. Then I can think of several intended audiences. A monster manual could have been written with the peasants in mind. Kind of a survival guide. Every monster would be seen as dangerous. An accounting of an adventure intended for other adventurers would have a different perspective. 'Watch out for this... Be careful about that... This is an opportunity...' A magical manual might have a much drier feel where facts are presented without trying to spin them one way or another. "Bog demons range from 35 ti 38 feet long."

Give me a moment. I'm going to go read a D&D book as if it was a found document. I'll come back with my conclusions.