There is an additional set of margins in a Pages document.

I exported a document to Pages '09 and opened it with the old version.  In the Layout Inspector, I found the "Layout Margins". The "Left" and "Before" margins were non-zero and the numbers matched the unwanted margins I saw in Pages 5.2.  Here's a sample:

![Layout Inspector from Pages '09][1]

I changed those layout margins to 0, saved the Pages '09 document, and opened it in Pages 5.2.  Hurrah!  The unwanted margins are gone!

If there's a UI control to view or change the Layout Margins in Pages 5.2, I haven't found it.

  [1]: https://i.sstatic.net/FJ2TO.png