It is probably the way it is because in the coding, back is the original page that launched (page 1). When you click on page 2, 3, and so on in a thread you're not actually moving to a new page, it's just updating the first page. A easy analogy, you're reading a book and your on page 1 and want to flip to the middle of the book and you change the page to page 84. You don't actually flip through 83 other pages to get there, page 1 just gets written with page 84 words in it.
If you don't, no biggie, it's a computer thing it is what it is, that's just how it works.