Most of the book data is imported from Amazon and the number of pages are often wrong since the data is usually entered long before the book is actually printed.
First verify that you shelved the same edition of the book that you are reading. Some books have many different editions in several formats. If the format (hardcover, paperback, trade paperback, etc.) and ISBN match, but the number of pages doesn’t match, the book data can be edited.
Goodreads does count indexes and end notes, but doesn’t count ads and extras like excerpts of the next book. WorldCat usually has the correct number, but not always.
To get the book data edited, just post a comment in the librarians group including a link to the book (the exact edition) and the correct number of pages. Other changes can be requested there too, such as bad descriptions or author issues.