The implication is that Trixia and the other translators are viewing Sherkaner and the rest of the Spiders and writing reports - this is important, it is why Sherkaner cannot reveal his plans to even Unnerby - of their activities.
The style Vinge uses is clearly something of a homage to Jules Verne and even moreso H.G. Wells. I think Vinge did this as part of a deliberate strategy, to make the reader identify with the Spiders as a race aspiring to technological progress, with the optimism often present in the early sci-fi works of the late-nineteenth and early-twentieth centuries. The writing deliberately lacks the sophistication one would expect of any sci-fi author after the nineteen-fifties. Since Vinge is a multiple-time Hugo winner - he won for that book, matter of fact - he clearly isn't the type to write Heinlein-style juvenile pulp from the 'fifties and 'sixties. Trixia's obsession with the "Dawn Age" - pre-interstellar flight Earth - gave Vinge an excellent excuse to use this style; this is what Trixia, and to a lesser extent the other translators, thinks of the Spiders.
As for the chapters in which the viewpoint shifts, this is usually just a case of Trixia writing - or orally translating, as in the case of the show Qwi organises - as things are also happening on the ships. Only in the very late stages of the book are the Spider chapters not being directly translated, as the Spiders have now become familiarised with humanity.