Ok example : I click on the page 5, the link will be "https://www.speedrun.com/the_site/forum/5" but i will be on the page 4.
When you are on page one, no number are highlight, and page 2 button go to page 1.
This is really weird.
That is correct, and this has already been reported before in this thread: https://www.speedrun.com/the_site/thread/yeqvv
This is most likely caused by some shenanigans due to 0-base and 1-base indexed arrays. It'd be great to get this fixed since it doesn't seem too complex and it really disrupts the flow of the site at some points.
PS: For now, when you need to go to page 2, click on number 3 instead. If it's the last page that you need, you can instead add a ".../n+1" to the URL, with n = the number of the last page.