As anyone who's seriously had a go at it will tell you, writing is an iterative process. The first draft is just the start. It will be revised, often many times, sometimes so many that the words you end up with bear very little resemblance to the words you originally wrote.
What I hadn't realised until I became a published author is how many more iterations are required between the point at which a book is accepted for publication and the day it hits the shelves. The editor may require changes; after this the ms will be copy-edited, resulting in changes made at a line level, and the author will need to check that over once it's done; and then proofs will be produced, which the author will again need to read through.
With Principles of Angels I was lucky: my editor only asked for a few minor changes before the copy-edit (and one afterwards, as a result of a point raised by an early reviewer). Consorts of Heaven, being that difficult second book, has required rather more work. Since delivering the book in June, I've done two significant lots of re-work: happily the fundamentals were there, and the story was essentially sound, but there was rather a lot of tweaking to be done. Earlier this week I got the copy-edited ms back, and have now gone through it again, checking the edits and making a few last small changes. I hope and believe that that's it and I'm all done ... until the page proofs.