Hi,
When you see param pagenumber3 it means its the pagenumber param for module id 3, its not a miscount of page numbers. This is in case there is more than one gallery on the page so they each have their own params.
However, I will be looking into the gallery soon and redo the paging using ajax instead of querystring params. The gallery was implemented a few years ago and could use some ajax love to make it more modern.
Best,
Joe