This is page numbering as distinguished from continuous sections. The start of a new section (required to switch to/from landscape) can restart page numbering. By default it does not do this. However, it seems that if you already restarted page numbering in the section where you add the section break, it will again restart.

In the new section (the one restarting when you do not want to restart), Insert > Page Number > Format page numbers > Continue from previous section.
You do not need to be inserting page numbers to change the formatting.

Page Numbers in Word 2007-2013