I can put it two ways, one of which sounds tautologically trivial: Everything looks different from everyone else's perspective. But a more literate way to put it may be this: Brust thinks there is no such thing as an omniscient viewpoint. And, possibly, that there is no objective reality.
We see this repeatedly, in the way different narrators cast the same events -- Between Brokedown Palace and The Phoenix Guard, the confrontation on the pepperfields; The Phoenix Guard again and Jhereg, in Aliera's and Paarfi's versions of Adron's Disaster. Did Morrolan accompany Zerika to Deathsgate Falls? Paresh and Paarfi tell us about the same events. Vlad meets Kiera when Sethra is confined to Dzur Mountain. Morrolan and Vlad hold separate conversations simultaneously with Verra.
In Tiassa, this was made overt to me when we accompany Cawti to see Norathar. When it is Vlad's viewpoint, in Yendi, she is nearly a nullity, because he is distracted by Cawti. We also get hints of Aliera's view of Norathar; from the eyes of a quintessential Dragon, Norathar is a pure Dragon. But here, when Cawti marches into the Dragon Wing to see her, Norathar is a girlfriend, lively and game to get up to something -- and far more on Vlad's side than Vlad would have believed possible.
Everything looks different when you see it through someone else's eyes. It's a strong signature for a writer; omniscient third and reliable first and their kin are such defaults for fiction that anything else stands out as a clear choice of style. But this goes beyond the unreliable narrator. Brust's characters aren't lying to the reader (much); they're not mistaken, or at least not in any way that's correctable. Brust's body of work asserts that viewpoint can't be reliable because reliability itself is an illusion.
Paarfi may be the clearest example of this. His Dumas style apery overwhelms the narrative. We know that the story cannot have occurred as Paarfi tells it; it is too artificial to be real. But, again, the default reader response is to believe that there is some reality underneath, that if you subtract off the style, you get to what happened. It is only in Brust's whole body of work that we realize that no, that's not at all what he intends. Vlad's more naturalistic narration is no closer to what happened than Paarfi's. Or, rather, narration -- or the nature of reality -- prevents anyone from saying what happened with objective authority.
It is a theme that demands a great deal of craft from the author. In each scene, Brust must not only give us one or more characters to follow through the scene, but must also determine how their viewpoint changes the scene itself. This is at the core of Teckla, of all things (and continues in Phoenix). Vlad and Cawti break up because their worlds are literally different. Not that they disagree, or that they no longer get along; they perceive different realities.
It has been a long time since I've read Brust's body of work outside Dragaera. But To Reign in Hell? What surprise is it that Brust would be drawn to a Miltonesque tale, where Lucifer's viewpoint so utterly denies God's -- that is, the definitively omniscient viewpoint.
There is no graspable reality in Brust's stories; there is only the truth that what happened depends on who's seeing it.