I think more specifically though, is that at least initially it kept pace age wise with its readers. Not everyone was maybe exactly the main characters age when they started reading it, but about the same amount of time had passed between them and the books became a bit more adult as a lot of the readers also got older, and this was also mirrored with the movies. Its like many of us actually grew up with the characters as they grew up.

That assumes that you write a book in a totally linear fashion instead of in various different threads and chapters and then weaves them together later. Writing a book from page 1 to the last page without changing or writing anything in between would be unusual. If he knew for sure that the first half of the book was what he wanted and it wouldn’t change at all as he wrote the second half, then yeah he could do that but he’d have to be very confident in where he was going already, in which case he’d probably just have it all done.

You joke, but seriously that could happen. People that don’t think very deeply about it will just equate page count to a progress meter and for a decent amount of the process that probably isn’t untrue, but near the end its the least likely to be relevant. This is the point where the writer is having to thread everything together and is going back to refine things they’ve already done. That could actually mean culling lines, pages, or even whole chapters for a variety of reasons.

I said else where but many creative people know that the last 10% of a project is often the most time consuming.