Since there are a number of PDF "pages" (the cover, the Table of Contents, etc.) before page "1" of a book, that explains the difference.
To change this, open Preview and then click on "Preview" at the top left of your screen. Next, select "Preferences...".
In the menu that appears, select the "PDF" icon on the right, and then tick "Use logical page numbers":
We have tested this and it worked for us, but we have also had a report that it did not work for someone.
More generally, regarding issues of this kind, please note that while we try to help where we can, there is a limit to the support we can supply when it comes to other companies' products, like Preview. Some issues need a level of technical support we can't provide ourselves, unfortunately.