Why does this sentence adopt different tenses?

Question

It was really lucky that Harry now had Hermlone as a friend. He didn't know how he'd have gotten through all his homework without her, what with all the last-minute Quidditch practice Wood was making them do. She had also lent him Quidditch Through the Ages, which turned out to be a very interesting read.
Harry learned that there were seven hundred ways of committing a Quidditch foul and that all of them had happened during a World Cup match in 1473; that Seekers were usually the smallest and fastest players, and that most serious Quidditch accidents seemed to happen to them; that although people rarely died playing Quidditch, referees had been known to vanish and turn up months later in the Sahara Desert.
(Harry Potter and the Sorcerer's Stone)

In a sentence, the first clause has past tense, and the second past perfect. What semantic difference comes from the different tenses?

Answer 1

Reference Time is ambiguous—it may refer to the time of writing Quidditch Through the Ages or it may refer to the time at which Harry read the work—but that particular sort of ambiguity is inherent when discussing written works. For practical purposes, RT may be taken to equate the two times, as if Harry in reading is 'listening to' the book.

So all the simple pasts occur in Harry's 'present', RT; the past perfects may represent either simple pasts (if the sense is perfective) or present perfects (if the sense is stative) in that time.

So what Harry reads is something like this

There are seven hundred ways of committing a Quidditch foul. All of them happened during a World Cup match in 1473. (The past perfect represents a backshifted simple past, referring to a completed event.)

Seekers are usually the smallest and fastest players, and most serious Quidditch accidents seem to happen to them.

People rarely die playing Quidditch, but referees have been known to vanish and turn up months later in the Sahara Desert. (The past perfect represents a backshifted present perfect representing a present state of knowledge.)

X has been known [by somebody] to VERB is, as Michel Plungjan points out, an idiom with a stative sense, meaning approximately [somebody] possesses knowledge of past instances of X VERBing. So that last clause means

... but there are cases on record of referees vanishing and turning up months later in the Sahara Desert.

X is known to VERB, in the present tense, has a somewhat different meaning: [somebody] possesses knowledge that X VERBs.

Answer 2

Most of this paragraph is written in the habitual past, meaning that the events described occur regularly. If Rowling had written:

referees vanished and turned up months later in the Sahara desert,

or

referees were known to vanish and turn up months later in the Sahara desert,

this would have implied that vanishing and turning up months later in the Sahara desert was something that happened fairly often to referees. I suspect the tense was changed to indicate that this was a relatively rare event.