What comes between the text and the res gesta is, in short, an act of narration ... figured as 'magic.' The historical world to be narrated is not immediately accessible -- in part because it is past ... in part because no configuration of facts, events, raw materials is thus accessible; historia is always narratio rerum gestarum, an instrument to apprehend the res gestae, which is clearly differentiated from the res gestae themselves. The material to be apprehended is and remains foreign and intractable: this is the uneasy insight dramatized by the otherwise 'pointless' otherworld anecdotes.
I find myself chewing over these lines as if I were an Ouroboros and they were my caudal appendage (to steal an awkward metaphor from the comments section). Actually, what I like about the passage is that it resists the temptation to turn the medieval historian into yet another example of "Phosphor Reading By His Own Light" (stealing Wallace Stevens now). Rather, Otter pushes twelfth-century writers into realms they seem to "uneasily" but vividly perceive, spaces where history as an impossible art is on magnificent display.
[Addendum 10/24: on re-reading my post about re-reading Otter, I see that I didn't point out something I disagree with in her book. Otter describes Wiliam of Malmesbury as ultimately trapped within his own vision of historical inadequacy: mournfully aware of the deadness of the past, its irretrievabilty. William of Newburgh she allows a more unsettled relation to a past that can't be so drained of life. I'd argue that both writers are creatively enagaged artists of history, who use their materials to revivify the past in unexpected, culturally impure, and temporally messy ways. See, for example, this post on Animal innovation in William of Malmesbury.]