Because 'sine' points to the subject, and the subject here is 'de'. You'd have to replace 'sine' with 'hennes' eller 'hans' respectively to achieve those two sentences.
"De leser bøkene hennes".
"They read her books."
"Hun leser bøkene sine."
"She reads her books." - her own books.
"Hun leser bøkene hennes."
"She reads her books." - books written or owned by some other woman.
The books are seriously!(en bok) means a book of Norwegian! geez! i have known of Norwegian!that was lame! Look at the Norwegian sentence! (et eple)means an apple of Norwegian!that replied me that on Norwegian! (hvem har en frukt) means who has a fruit of Norwegian!why does (et egg) mean of Norwegian? that means an egg of Norwegian! that was so dumb!geez bro! (hvem har et brev og vann) means who has the letter and water of Norwegian! (Hva spiser hun) means what is she writing of Norwegian!
The vocab list in the tips and notes section for this skill doesn't talk about si/sin/sine. The example they give for "their" is deres. What's the difference between si/sin/sine and deres? I've looked at the comments for other sentences (like Mannen elsker jenta si) and people have said that it's because using "dere" means that the man loved someone else's girlfriend, but what about this sentence? Does "sine" here mean that the people are reading books that belong to them, or books that they themselves have written? Why do the tips and notes say to use "deres" and then never actually give any sentences that use that word in the lessons?