How to figure out button destination list in InDesign file

I have an InDesign file that has buttons on the top of the page.  When you export it as a PDF interactive, the buttons allow you to navigate throughout the document. I know how to update the buttons so a button goes to a different page within the document.  But when I select Destination and the drop down comes on it lists names of various headers on the page.  FYI - I did not design this document as it was given to me by the designer and now we're left to updating it ourselves.

The pages in the document are numbered as they usually are in the pages panel (e.g., 1,2,3 etc.)

But this new page that was added in (page #18) id not actually showing up in the destination drop down of the buttons panel. I'm guessing here but perhaps there's some kind anchor on each page that was named for each page so perhaps that's what the destination drop down list is displaying vs the page numbers - obviously it'd make it much easier if the destination was showing the page numbers and I'd easily select the one to go to.

I'm sorry I can't give more info here and I'm hoping that this makes sense to someone and would help me figure this out. Below is what the destination panel in buttons looks like.

Any help is appreciated.  Thanks.