Why did my final page count change?

Your Word document's page count was based on Word's page size, font, and spacing. The formatter throws all of that out and lays your book out at its real trim size with book typography — different typeface, leading, margins, chapter openers that start fresh pages, recto starts with the occasional blank, and widow/orphan control.

The result is a true finished page count that will almost always differ from your Word count — often it's what the printed book will actually be. This is expected and correct, not an error.

The one thing to do with the new number: make sure your cover's spine matches it. The formatter writes the page count back to your cover — see Does the new page count update my cover?